libmount: revert X-* and x-* meaning

libmount: revert X-* and x-* meaning

Let's hope this is last change necessary to cleanup x-* usage:

  x-*  persistent option, stored in utab, available for umount, etc.
  X-*  fstab comment only

mount(8) supports x-mount.mkdir= as well as newly recommended X-mount.mkdir=

Advantages:

 * less invasive
 * does not require exception for x-systemd
 * does not require rename x-initrd to X-initrd

The systemd and dracut users will get the new (=fixed) functionality without a
change in fstab configuration. This is the primary goal.

Disadvantages:

 * not 100% compatible libmount behavior, x-* options have not been
   previously stored in utab. The API is the same, options will be still
   available, but on x-* libmount will write to /run/mount/utab. For now
   it seems only systemd uses x-*, and they like this behavior, so...

Addresses: https://github.com/systemd/systemd/pull/4515
Signed-off-by: Karel Zak <kzak@redhat.com>