Represent logged in time in seconds rather than minutes

This simplifies things.  In particular, it gets rid of random rounding
in getcommand(), which created a variation in the nightly build
depending on whether the update starts before or after the deity logs
out.

Replace struct natstr member nat_minused by nat_timeused, and update
cou_ca[] accordingly (this affects xdump nat).  Replace player member
minleft by timeleft, and getminleft() by gettimeleft().  Update
getcommand(), daychange(), player_main(), status() accordingly, taking
care not to change player output.  Change edit country key 'u' to work
in seconds.

src/lib/subs/natsub.c

@@ -85,7 +85,7 @@ nat_reset(struct natstr *natp, enum nat_status stat, coord x, coord y)
     }
 
     natp->nat_dayno = 0;
-    natp->nat_minused = 0;
+    natp->nat_timeused = 0;
     natp->nat_update = 0;
 
     natp->nat_tgms = 0;