Support starting containers concurrently

Trying to start multiple containers concurrently may cause lxc_monitor_read_timeout to fail as select call could be interrupted by a signal, handle it.
Support stopping containers concurrently

Trying to stop multiple containers concurrently ends up with "cgroup is not mounted" errors as multiple threads corrupts the shared variables.
Fix that stack corruption and start to use getmntent_r to support stopping multiple containers concurrently.