git issue #838 - old time manipulation for BSD

Issue #838 from clausecker, relayed by copperwater:  old workarounds
for lack of type 'time_t' from pre-standard days aren't suitable any
more.  One of the instances was incorrect (diagnosed by entrez) and
no one had noticed for years (or possibly just ignored a compiler
warning).

Remove most of the old cruft from hacklib.c and some from system.h
but put in commented workarounds in unixconf.h in case someone needs
to resurrect it.  It would have been better to do things this way
back in the old days.

Resurrecting some non-Unix port might need to clone the unixconf.h
bits in its own *conf.h, but that probably won't be necessary for a
standard C compliant system.

Closes #838

doc/fixes3-7-0.txt

@@ -2092,4 +2092,6 @@ add TT_NONE==0, renumber other u.utraptype values so that TT_BEARTRAP isn't 0
 for Planes of Water and Air, save the air bubbles and clouds with the level
 	rather than as game state; affects wizard mode ^V and #wizmakemap
 switch to a flags arg on newcham() in place of the pair of boolean args
+replace some old 'time_t' hackery in system.h and hacklib.c with something
+	less obtrusive in unixconf.h