chns: Better Ubuntu support

Ubuntu's /etc/mtab is not a simbolic link of /proc/mounts and
it's overlayfs module doesn't loaded until trying to mount.
This commit prepares 3 things:
 - set LIBMOUNT_MTAB to /proc/mounts, this avoids updating /etc/mtab
   for private overlayfs mount.
 - Forcibly try to mount overlay at first. If it failed, tries overlayfs
   again.
 - override /etc/mtab to /proc/mounts on the overlayfs, if needed.

This allows us to run chns in vanilla Ubuntu 14.10.