elapsed time handling

The code has been assuming that time_t is some number of seconds.
That's valid for traditional Unix systems and for Posix compliant
systems but is not something guaranteed by the C standard.  (We ran
into a long time ago when trying out an alternate way to calculate
phase of moon.  That code made a similar assumption and broke one
of the ports.)

'ubirthday' also warrants being re-done but I've run out of energy.

src/end.c

@@ -1273,7 +1273,7 @@ really_done(int how)
        topten figure it out separately and possibly getting different
        time or even day if player is slow responding to --More-- */
     urealtime.finish_time = endtime = getnow();
-    urealtime.realtime += (long) (endtime - urealtime.start_timing);
+    urealtime.realtime += timet_delta(endtime, urealtime.start_timing);
     /* collect these for end of game disclosure (not used during play) */
     iflags.at_night = night();
     iflags.at_midnight = midnight();