libmount: improve X-mount.mkdir for non-root users

libmount: improve X-mount.mkdir for non-root users

Since v2.35 mount(8) drops suid on -EPERM and repeat necessary actions
before mount(2) syscall. This patch also improves this behavior for
X-mount.mkdir too.

mount(8):
 * return -EPERM on sanitize_paths() rather than call err()
 * call suid_drop() on failed sanitize_paths()
 * update man page

libmount:
 * mnt_context_prepare_target() refactoring
 * return -EPERM when in restricted mode for X-mount.mkdir

Fixed version:
 /home/kzak/mnt-foo   sr.net.home:/home/kzak   fuse.sshfs noauto,X-mount.mkdir

 $ mount /home/kzak/mnt-foo
 kzak@sr.net.home's password:

 $ /home/projects/util-linux/util-linux  findmnt /home/kzak/mnt-foo
 TARGET             SOURCE                 FSTYPE     OPTIONS
 /home/kzak/mnt-foo sr.net.home:/home/kzak fuse.sshfs rw,nosuid,nodev,relatime,user_id=1000,group_id=1000

Addresses: https://github.com/systemd/systemd/issues/14418
Signed-off-by: Karel Zak <kzak@redhat.com>