libmount: don't support /etc/mtab by default

libmount: don't support /etc/mtab by default

The file mtab is evil and already unused by mainstream distributions.

Now libmount is able to detect mtab->/proc/mounts and use
/proc/self/mountinfo if necessary. This heuristic seems overkill in
many cases. It's also dangerous on systems where mountinfo is strongly
required (systemd based distros).

This patch #ifdefs mtab code and forces libmount to always use
/proc/self/mountinfo.

The new configure option --enable-libmount-support-mtab is necessary
to enable old behavior to support mtab.

Signed-off-by: Karel Zak <kzak@redhat.com>