Container: Always create a new session for the container

We never want the container to be in the same process group as the
daemon, as then the container will receive signals sent to the
process group of the container.
Merge pull request #1989 from alexlarsson/setsid

Container: Always create a new session for the container
fix the error message so it is the same as the regex issue #1999
Merge pull request #2004 from kencochrane/master

fix the error message so it is the same as the regex issue #1999