libmount: make mnt_context_is_fs_mounted work for /proc

libmount: make mnt_context_is_fs_mounted work for /proc

Assume that /proc is not mounted instead of returning an error when we
are unable to open the mounts and mountinfo files in /proc. Also set
cxt->mtab back to NULL so that it gets properly parsed when we check if
the next filesystem is mounted.

The goal is to have mount -a work when /proc is not mounted, typically
with /proc on the first line of fstab.

Signed-off-by: Ivan Delalande <colona@arista.com>