Fix regression of exec command
Fix regression of exec command

Set minimal required args to 1 for `exec` command to roll back to
previous behavior, also modify help message a little bit.

Signed-off-by: Zhang Wei <14976e959b2860fb06a3c1edbdb1945a4e11b8b5@huawei.com>

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