mount: no exit on EPERM, continue without suid

mount: no exit on EPERM, continue without suid

The current libmount assumes that mount(8) and umount(8) are suid
binaries. For this reason it implements internal rules which
restrict what is allowed for non-root users. Unfortunately, it's
out of reality for some use-cases where root permissions are no
required. Nice example are fuse filesystems.

So, the current situation is to call exit() always when mount, umount or
libmount are unsure with non-root user rights. This patch removes the
exit() call and replaces it with suid permissions drop, after that it
continues as usually. It means after suid-drop all depend on kernel
and no another security rule is used by libmount (simply because any
rule is no more necessary).

Example:

old version:
   $ mount -t fuse.sshfs kzak@192.168.111.1:/home/kzak /home/kzak/mnt
   mount: only root can use "--types" option

new version:
   $ mount -t fuse.sshfs kzak@192.168.111.1:/home/kzak /home/kzak/mnt
   kzak@192.168.111.1's password:

   $ findmnt /home/kzak/mnt
   TARGET         SOURCE                        FSTYPE     OPTIONS
   /home/kzak/mnt kzak@192.168.111.1:/home/kzak fuse.sshfs rw,nosuid,nodev,relatime,user_id=1000,group_id=1000

   $ umount /home/kzak/mnt
   $ echo $?
   0

Note that fuse user umount is supported since v2.34 due to user_id= in
kernel mount table.

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