libmount: introduce X-* comments

libmount: introduce X-* comments

x-* are fstab only and not stored in userspace (utab). In some cases
it's not optional solution because the option is also necessary for
umount or another operations. The X-* is exactly the same as x-*, but
stored to utab (or mtab on old systems).

It's usually bad idea to store mount options in userspace, but it's
better to provide any solution that force users to bypass mount(8)
(and friends) and implement 3rd-party incompatible solutions.

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