Generate symlink /etc/mtab pointing to /proc/mounts [Closes: issue1277]

Generate symlink /etc/mtab pointing to /proc/mounts [Closes: issue1277]

We don't have /etc/mtab on our rootfs by default and Debian's
initscript package provides a way to generate the /etc/mtab
symlink via its /etc/init.d/checkroot.sh init script. We don't
use this init script in Grml's live system though (e.g. to not
enable swap partitions by default).

Also /etc/init.d/mountkernfs.sh creates an *empty* /etc/mtab file
on bootup if /etc/mtab doesn't exist yet. Last but not least
live-boot's /lib/live/boot/9990-aaa-fixme.sh as well as brltty's
/scripts/init-premount/brltty [see #724695] do some further hacks
- so let's just force generation of symlink /etc/mtab pointing to
/proc/mounts to make sure the system is in a sane state.

While at it drop deprecated /live/cow related code for /etc/mtab
handling.

Thanks: Michael Biebl for the initial hint
regarding the "df: no file systems processed" issue