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.

src/lib/subs/natsub.c

@@ -84,7 +84,6 @@ nat_reset(struct natstr *natp, enum nat_status stat, coord x, coord y)
 	natp->nat_yorg = 0;
     }
 
-    natp->nat_dayno = 0;
     natp->nat_timeused = 0;
     natp->nat_update = 0;