nsenter: major cleanup
nsenter: major cleanup

Removed a lot of clutter, improved the style of the code, removed
unnecessary complexity. In addition, made errors unique by making bail()
exit with a unique error code. Most of this code comes from the current
state of the rootless containers branch.

Signed-off-by: Aleksa Sarai <a8eb12d32d6ec07cdf737175ec7d4a0f1b4e1ee7@suse.de>

maxamillion/runc,kolyshkin/runc,kolyshkin/runc,WeiZhang555/runc,rajasec/runc,opencontainers/runc,dqminh/runc,estesp/runc,docker/runc,rhatdan/runc,coolljt0725/runc,docker/runc,opencontainers/runc,rhatdan/runc,avagin/runc,shishir-a412ed/runc,xiaochenshen/runc,nkwilson/runc,justincormack/runc,xiaochenshen/runc,hqhq/runc,adrianreber/runc,justincormack/runc,rhatdan/runc,dqminh/runc,stefanberger/runc,sunyuan3/runc,crosbymichael/runc,vbatts/runc,justincormack/runc,mikebrow/runc,hqhq/runc,crosbymichael/runc,mheon/runc,nkwilson/runc,crosbymichael/runc,coolljt0725/runc,cloudfoundry-incubator/runc,jhowardmsft/runc,opencontainers/runc,dqminh/runc,hqhq/runc,wking/runc,maxamillion/runc,mikebrow/runc,mheon/runc,stefanberger/runc,docker/runc,wking/runc,cyphar/runc,shishir-a412ed/runc,laijs/runc,adrianreber/runc,xiaochenshen/runc,sunyuan3/runc,nkwilson/runc,mYmNeo/runc,WeiZhang555/runc,wking/runc,dqminh/runc,kolyshkin/runc,vbatts/runc,estesp/runc,keloyang/runc,mYmNeo/runc,WeiZhang555/runc,cloudfoundry-incubator/runc,rajasec/runc,shishir-a412ed/runc,avagin/runc,maxamillion/runc,coolljt0725/runc,cyphar/runc,avagin/runc,estesp/runc,jhowardmsft/runc,stefanberger/runc,mikebrow/runc,laijs/runc,rhatdan/runc,mYmNeo/runc,adrianreber/runc,rajasec/runc,vbatts/runc,cyphar/runc,sunyuan3/runc,keloyang/runc,mheon/runc,laijs/runc,opencontainers/runc,keloyang/runc,kolyshkin/runc,jhowardmsft/runc,cloudfoundry-incubator/runc