libuuid: reset initial cont-clock time on service start

libuuid: reset initial cont-clock time on service start

When uuidd starts with --cont-clock enabled and reads a stale timestamp
from the state file, the continuous clock logic would apply a backwards
offset (default 2 hours), causing generated UUIDs to have incorrect
timestamps in the past.

Fix this by resetting last_clock_reg to the current time when the saved
timestamp is older than the current clock. This prevents the backwards
offset from being applied at service start.

[kzak@redhat.com: - fix comment, add commit message]

Fixes: https://github.com/util-linux/util-linux/issues/4018
Signed-off-by: Karel Zak <kzak@redhat.com>