cgroup: address some style+safety issues
cgroup: address some style+safety issues

three issues raised by Christian Seiler: use pid_t, use snprintf,
and use const.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

weizhenwei/lxc,sapun/lxc,thmo/lxc,glensc/lxc,major/lxc,glensc/lxc,hkjolhede/lxc,bostjan/lxc,armcc/lxc,rowhit/lxc,Pelagicore/lxc,regit/lxc,hfuCN/lxc,szaszg/lxc,bmoar/lxc,ss1h2a3tw/lxc,JohnPeacockMessageSystems/lxc,regit/lxc,ojkastl/lxc_Patches,terceiro/lxc,virajs/lxc,raspberrypython/lxc,weizhenwei/lxc,hallyn/lxc,ioops/lxc,Pelagicore/lxc,codebauss/lxc,yubo/lxc,ojkastl/lxc_Patches,lazy404/lxc,cjwatson/lxc,hallyn/lxc,ecarrara/lxc,QingweiPeterLan/cs188-lxc,virajs/lxc,terceiro/lxc,ysbnim/lxc,major/lxc,ioops/lxc,bostjan/lxc,caglar10ur/lxc,lazy404/lxc,hallyn/lxc,QingweiPeterLan/cs188-lxc,efiop/lxc,christiaan/lxc,rowhit/lxc,cjwatson/lxc,akshaykarle/lxc,rldleblanc/lxc,federicobriata/lxc,jirislaby/lxc,ecarrara/lxc,jirislaby/lxc,rowhit/lxc,sapun/lxc,ysbnim/lxc,szaszg/lxc,weizhenwei/lxc,GreatFruitOmsk/lxc,bmoar/lxc,Ponce/lxc,hfuCN/lxc,Distrotech/lxc,efiop/lxc,virajs/lxc,tych0/lxc,thmo/lxc,federicobriata/lxc,akshaykarle/lxc,fwilson42/lxc,ss1h2a3tw/lxc,Distrotech/lxc,ysbnim/lxc,cjwatson/lxc,codebauss/lxc,yubo/lxc,raspberrypython/lxc,akshaykarle/lxc,regit/lxc,DarknessBeforeDawn/lxc,GreatFruitOmsk/lxc,szaszg/lxc,christiaan/lxc,ojkastl/lxc_Patches,terceiro/lxc,ksperis/lxc,christiaan/lxc,jirislaby/lxc,tych0/lxc,efiop/lxc,raspberrypython/lxc,armcc/lxc,LynxChaus/lxc,ksperis/lxc,hkjolhede/lxc,Ponce/lxc,bostjan/lxc,Pelagicore/lxc,sapun/lxc,fwilson42/lxc,DarknessBeforeDawn/lxc,ioops/lxc,GreatFruitOmsk/lxc,fwilson42/lxc,tych0/lxc,yubo/lxc,caglar10ur/lxc,LynxChaus/lxc,lisongmin/lxc,bmoar/lxc,lisongmin/lxc,rldleblanc/lxc,ksperis/lxc,LynxChaus/lxc,lisongmin/lxc,QingweiPeterLan/cs188-lxc,ecarrara/lxc,major/lxc,aeris/lxc,hfuCN/lxc,rldleblanc/lxc,caglar10ur/lxc,thmo/lxc,federicobriata/lxc,lazy404/lxc,DarknessBeforeDawn/lxc,JohnPeacockMessageSystems/lxc,JohnPeacockMessageSystems/lxc,ss1h2a3tw/lxc,Ponce/lxc,hkjolhede/lxc,glensc/lxc,armcc/lxc,aeris/lxc,aeris/lxc,Distrotech/lxc,codebauss/lxc,ojkastl/lxc_Patches