mkswap: set UUID for swap space (add -U option)

mkswap: set UUID for swap space (add -U option)

A friend of mine is looking into the possibility of cloning Debian
(and other) systems automatically and stomped over swap partitions
getting assigned new UUIDs every time the new harddisk is partitioned
and swap is created.

It's essential when partitions are to be recognised by their uuid and
not by their old device path anymore.

Addresses-Ubuntu-Bug: #66637
Signed-off-by: LaMont Jones <lamont@debian.org>
Signed-off-by: Karel Zak <kzak@redhat.com>