mount: document x-* options

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount; add recursive mkdir

Signed-off-by: Karel Zak <kzak@redhat.com>
mount: add support for x-mount.mkdir[=<mode>] option

If the target directory (mountpoint) does not exist then mount(8) will create
it before mount.<type> is executed or mount(2) syscall is called.

Co-Author: Karel Zak <kzak@redhat.com>
Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>