Provide an Empire clock with ETU resolution that is properly

synchronized to updates (the one based on s_p_etu isn't synchronized):
(gamestr): New members game_turn, game_tick, game_rt.
(game_ca): New selectors for them.  Restrict tick and rt to deity for
now.  We'd need xdump update the Empire clock to make them safe for
players.
(game_record_update): New.
(update_main): Call it.
(game_tick_tick, game_tick_to_now, game_step_a_tick): New, not yet
used.

src/lib/global/nsc.c

@@ -545,6 +545,10 @@ struct castr realm_ca[] = {
 };
 
 struct castr game_ca[] = {
+    {NSC_SHORT, 0, 0, offsetof(struct gamestr, game_turn), "turn", EF_BAD},
+    {NSC_SHORT, NSC_DEITY, 0, offsetof(struct gamestr, game_tick), "tick",
+     EF_BAD},
+    {NSC_TIME, NSC_DEITY, 0, offsetof(struct gamestr, game_rt), "rt", EF_BAD},
     {NSC_NOTYPE, 0, 0, 0, NULL, EF_BAD}
 };