util: fix and move get_current_time_msec into a util file

This commit makes `get_current_time_msec` correctly return milliseconds
as opposed to microseconds. It also considers the value of `tv_sec`, so
we don't lose occasionally go back in time by one second. Finally, the
function is moved into `util/time.cc` so that it can be reused elsewhere
without having to consider these pitfalls.

util/meson.build

@@ -5,4 +5,5 @@ wlr_files += files(
 	'region.c',
 	'shm.c',
 	'signal.c',
+	'time.c',
 )