Remove "root" and "" special cases in libcontainer
Remove "root" and "" special cases in libcontainer

These are unnecessary since the user package handles these cases properly already (as evidenced by the LXC backend not having these special cases).

I also updated the errors returned to match the other libcontainer error messages in this same file.

Also, switching from Setresuid to Setuid directly isn't a problem, because the "setuid" system call will automatically do that if our own effective UID is root currently: (from `man 2 setuid`)

    setuid() sets the effective user ID of the calling process.  If the
    effective UID of the caller is root, the real UID and saved set-user-
    ID are also set.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

SaiedKazemi/libcontainer-criu,balajibr/runc,adrianreber/runc,hxfeng/runc,sallyom/runc,abduld/runc,whosthatknocking/runc,LTD-Beget/libcontainer,jwuwork/runc,maxamillion/runc,YujiOshima/runc,lizf-os/runc,kimh/runc,cyphar/runc,LK4D4/runc,axsh/runc,keloyang/libcontainer,hxfeng/runc,rhatdan/libcontainer,mrunalp/runc,rajasec/runc,vbatts/runc,rsampaio/runc,dieface/runc,icecrime/libcontainer,markllama/runc,ycaihua/runc,milosgajdos83/libcontainer-milosgajdos83,mapk0y/runc,boucher/libcontainer,rhatdan/runc,Microsoft/libcontainer,estesp/runc,stefanberger/runc,mheon/libcontainer,jhjeong-kr/runc,avagin/libcontainer,ekuric/runc,rootfs/runc,runcom/libcontainer,ChengTiesheng/runc,chenchun/runc,adrianreber/runc,cloudfoundry-incubator/runc,rsampaio/runc,WeiZhang555/runc,sallyom/runc,justincormack/runc,boucher/runc,ZJU-SEL/libcontainer,mrunalp/libcontainer,marcosnils/runc,mYmNeo/runc,keloyang/libcontainer,Mashimiao/libcontainer,aidanhs/runc,vbatts/runc,navidshaikh/runc,dqminh/runc,clnperez/runc,talwai/runc,mheon/runc,intfrr/runc,abduld/runc,estesp/libcontainer,SaiedKazemi/libcontainer-criu,clnperez/runc,ZJU-SEL/libcontainer,shenxiaochen/libcontainer,rhatdan/libcontainer,mrunalp/libcontainer,meihuabo/runc,rootfs/runc,runcom/runc,willmtemple/runc,jhowardmsft/runc,zhangyancoder/libcontainer,maxamillion/runc,runcom/runc,hqhq/libcontainer,InformatiQ/runc,willmtemple/runc,opencontainers/runc,jhjeong-kr/runc,rajasec/runc,talwai/runc,InformatiQ/runc,docker/runc,albertoleal/runc,HardikDR/runc,gitido/runc,mrunalp/runc,albertoleal/runc,subuser-security/runc,meihuabo/runc,Microsoft/libcontainer,jhowardmsft/runc,ZJU-SEL/libcontainer,keloyang/libcontainer,ycaihua/libcontainer,shishir-a412ed/runc,cloudfoundry-incubator/runc,kolyshkin/runc,avagin/runc,mYmNeo/runc,wcwxyz/runc,HardikDR/runc,crosbymichael/libcontainer,inatatsu/runc,subuser-security/runc,mikebrow/runc,axsh/runc,rajasec/runc,gitido/runc,nkwilson/runc,docker/runc,laijs/runc,dqminh/runc,hqhq/libcontainer,shenxiaochen/libcontainer,estesp/libcontainer,shakamunyi/libcontainer,calavera/runc,HuKeping/runc,ddnbgroup/runc,mheon/runc,mapk0y/runc,adrianreber/runc,carlye566/runc,wangkirin/runc,fabiokung/runc,marcosnils/runc,zenlinTechnofreak/runc,ekuric/runc,subuser-security/runc,nkwilson/runc,runcom/libcontainer,xiaochenshen/runc,ChengTiesheng/runc,cyphar/runc,icecrime/libcontainer,lizf-os/runc,crosbymichael/runc,aidanhs/runc,cxxly/runc,Microsoft/libcontainer,hqhq/libcontainer,wangkirin/runc,docker/libcontainer,carlye566/runc,boucher/runc,estesp/runc,booyaa/runc,ycaihua/runc,avagin/libcontainer,crosbymichael/runc,opencontainers/runc,boucher/libcontainer,mrunalp/libcontainer,avagin/runc,fabiokung/runc,mheon/runc,crosbymichael/libcontainer,dqminh/runc,w4ngyi/libcontainer,booyaa/runc,gwoo/runc,kolyshkin/runc,xiaochenshen/runc,hqhq/runc,BrickXu/libcontainer,balajibr/runc,thaJeztah/libcontainer,HuKeping/runc,dieface/runc,jwuwork/runc,cyphar/runc,Mashimiao/libcontainer,unclejack/libcontainer,inatatsu/runc,opencontainers/runc,markllama/runc,mikebrow/runc,dqminh/libcontainer,unclejack/libcontainer,inatatsu/runc,chenchun/runc,HardikDR/runc,ddnbgroup/runc,thaJeztah/libcontainer,HuKeping/runc,LTD-Beget/libcontainer,kolyshkin/runc,tangkun75/libcontainer,docker/libcontainer,rhatdan/runc,mapk0y/runc,liuwharton/runc,clnperez/runc,runcom/libcontainer,rhatdan/runc,keloyang/libcontainer,ycaihua/runc,InformatiQ/runc,navidshaikh/runc,balajibr/runc,SaiedKazemi/libcontainer-criu,runcom/runc,abduld/runc,fabiokung/runc,opencontainers/runc,ZJU-SEL/runc,rhatdan/runc,shishir-a412ed/runc,mheon/libcontainer,docker/libcontainer,liuwharton/runc,wangkirin/runc,haijunTan/runc,shishir-a412ed/runc,zenlinTechnofreak/runc,talwai/runc,xiaochenshen/runc,hqhq/runc,mYmNeo/runc,YujiOshima/runc,chenchun/runc,crosbymichael/runc,zhangyancoder/libcontainer,zenlinTechnofreak/runc,dqminh/libcontainer,hqhq/runc,stefanberger/runc,docker/runc,YujiOshima/runc,tangkun75/libcontainer,WeiZhang555/runc,cloudfoundry-incubator/runc,icecrime/libcontainer,booyaa/runc,willmtemple/runc,keloyang/runc,unclejack/libcontainer,ZJU-SEL/runc,keloyang/runc,crosbymichael/libcontainer,zhangyancoder/libcontainer,thaJeztah/libcontainer,estesp/libcontainer,whuwxl/libcontainer,calavera/runc,carlye566/runc,axsh/runc,ChengTiesheng/runc,whosthatknocking/runc,wcwxyz/runc,rsampaio/runc,calavera/runc,kimh/runc,coolljt0725/runc,haijunTan/runc,avagin/libcontainer,sallyom/runc,LK4D4/runc,ekuric/runc,stefanberger/runc,haijunTan/runc,liuwharton/runc,whuwxl/libcontainer,kolyshkin/runc,meihuabo/runc,maxamillion/runc,whosthatknocking/runc,boucher/libcontainer,mrunalp/runc,milosgajdos83/libcontainer-milosgajdos83,cxxly/runc,sunyuan3/runc,tangkun75/libcontainer,BrickXu/libcontainer,dqminh/runc,w4ngyi/libcontainer,intfrr/runc,nkwilson/runc,rhatdan/libcontainer,wking/runc,shakamunyi/libcontainer,sunyuan3/runc,justincormack/runc,han-wang/runc,laijs/runc,coolljt0725/runc,Mashimiao/libcontainer,shakamunyi/libcontainer,rootfs/runc,vbatts/runc,w4ngyi/libcontainer,marcosnils/runc,LK4D4/runc,ddnbgroup/runc,mikebrow/runc,keloyang/runc,dqminh/libcontainer,BrickXu/libcontainer,jhjeong-kr/runc,intfrr/runc,jhowardmsft/runc,ycaihua/libcontainer,jwuwork/runc,gitido/runc,wking/runc,hxfeng/runc,wcwxyz/runc,estesp/runc,laijs/runc,dieface/runc,ycaihua/libcontainer,boucher/runc,aidanhs/runc,cxxly/runc,wking/runc,kimh/runc,coolljt0725/runc,WeiZhang555/runc,han-wang/runc,albertoleal/runc,whuwxl/libcontainer,shenxiaochen/libcontainer,ZJU-SEL/runc,navidshaikh/runc,avagin/runc,lizf-os/runc,han-wang/runc,LTD-Beget/libcontainer,mheon/libcontainer,justincormack/runc,markllama/runc,sunyuan3/runc