hangup() cleanup; SIGXCPU handling (trunk only)

While looking at a bug report from two months ago about how having
hangup take place while at the "destroy old game?" prompt causes an
interrupted game to become unrecoverable, I decided to try to clean up
convoluted hangup() a bit.  Didn't achieve much there, but did notice that
the Unix port was handling SIGXCPU (whatever that is...) inconsistently.
This attempts to fix that, but I have no way to test it.

     The original bug report is still unresolved.

     makedefs ought to add something for SAFERHANGUP and NOSAVEONHANGUP
to the #version output.

[ This is the missing bit from the patch on Monday which accidentally
  included all of extern.h as its log entry message. ]

include/extern.h

@@ -2277,6 +2277,7 @@ E int FDECL(flash_hits_mon, (struct monst *,struct obj *));
 # ifdef PORT_HELP
 E void NDECL(port_help);
 # endif
+E void FDECL(sethanguphandler, (void (*)(int)));
 #endif /* UNIX */
 
 
@@ -2405,6 +2406,7 @@ E int FDECL(main, (int, char **));
 # ifdef CHDIR
 E void FDECL(chdirx, (const char *,BOOLEAN_P));
 # endif /* CHDIR */
+E void FDECL(sethanguphandler, (void (*)(int)));
 
 /* ### vmsmisc.c ### */