grml-chroot: iff /proc/cmdline exists don't try to mount again

There's no point in mounting /proc, /sys, /dev + /dev/pts
once again if it's mounted already. The easiest and cheapest
way to detect this is looking for file /proc/cmdline. If
/proc/cmdline exists assume that the chroot has the filesystems
mounted already.

Closes: https://github.com/grml/grml-scripts/issues/4