Rewrite accounting of play time

Replace daychange() and gettimeleft() by update_timeused_login(),
update_timeused() and enforce_minimum_session_time().  The new
code doesn't assume the day is always 24 hours long which can
occur when transitioning into or out of DST and such.  Logging
in after more a multiple of 128 days now resets nat_timeused
properly.

Fix nat_timeused calculation on midnight rollover to include
the time since midnight.

struct natstr member nat_dayno and struct player member timeleft
are now unused, remove them.

include/player.h

@@ -67,8 +67,7 @@ struct player {
     char *argp[128];		/* arguments, ASCII, valid if command */
     char *condarg;		/* conditional, ASCII, valid if command */
     char *comtail[128];		/* start of args in combuf[] */
-    time_t lasttime;		/* when timeleft was last debited */
-    int timeleft;
+    time_t lasttime;		/* when nat_timeused was last updated */
     int btused;
     int god;
     int owner;