libmount: add support for ID=

libmount: add support for ID=

This patch add support for a new tag. The tag is based on udev block
device ID (see /dev/disk/by-id). The usual use-case is to use
WWN for this purpose, for example

  # mount ID=wwn-0x50026b724b09a1ff /mnt

Note that ID is not strictly defined and udevd generates various IDs
also for HW where WWN is undefined. This is reason why introduce ID=
seems better and more generic than more restrictive WWN=.

Addresses: https://github.com/karelzak/util-linux/issues/1008
Signed-off-by: Karel Zak <kzak@redhat.com>