Core Update 167: Replace /etc/mtab by symlink to /proc/self/mounts

Core Update 167: Replace /etc/mtab by symlink to /proc/self/mounts

mount, as updated via util-linux, no longer writes /etc/mtab, causing
programs to rely on this file's content (such as the check_disk Nagios
plugin) to stop working.

/proc/self/mounts contains all the necessary information, so it is fine
to replace /etc/mtab by a symlink to it.

Fixes: #12843
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>