uuidd: fix /var/lib/libuuid mode uuidd-tmpfiles.conf

uuidd: fix /var/lib/libuuid mode uuidd-tmpfiles.conf

The directory can be directly used by libuuid, for example, by running
"uuidgen --time" as root, even if the uuidd daemon is not installed.
In this case, we must create a clock.txt file with the uuidd group to
prevent any potential ownership mismatch if the uuidd daemon is
installed later. The 2xxx mode (setgid) forces open(O_CREAT) in
libuuid to create the file with the uuidd group.

Please note that mode 2755 is the standard used for years in RHEL.

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