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/nat.h

@@ -95,7 +95,6 @@ struct natstr {
     char nat_userid[32];	/* userid of last user, may be empty */
     coord nat_xcap, nat_ycap;	/* cap location in abs coords */
     coord nat_xorg, nat_yorg;	/* origin location in abs coords */
-    signed char nat_dayno;	/* day of the year mod 128 */
     signed char nat_update;	/* Want an update or not. */
     unsigned short nat_tgms;	/* # of telegrams to be announced */
     unsigned short nat_ann;	/* # of annos pending */