libmount: Add API to get/set unique IDs

libmount: Add API to get/set unique IDs

Since the Linux kernel version 6.8, there are two types of IDs
available: the "old" ID used in /proc/self/mountinfo, and a new 64-bit
unique ID that is never recycled. This new ID is provided by the
statx(STATX_MNT_ID_UNIQUE) and statmount() syscalls.

Note that this patch only adds the API for retrieving these unique
IDs, but the backing code has not been implemented yet.

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