common/util: fix get_current_time_msec returning microseconds

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.c` so that it can be reused elsewhere
without having to consider these pitfalls.

include/util.h

@@ -6,7 +6,13 @@
 #include <wayland-server-protocol.h>
 
 /**
- * Wrap i into the range [0, max[
+ * Get the current time, in milliseconds.
+ */
+
+uint32_t get_current_time_msec(void);
+
+/**
+ * Wrap i into the range [0, max]
  */
 int wrap(int i, int max);