more nethack -s

For Unix, set plname[] to the default value (player's username)
before running prscore() for 'nethack -s'.  Avoids reference to
mysterious "hackplayer" if no entries are found.

sys/unix/unixmain.c

@@ -731,6 +731,7 @@ scores_only(int argc, char **argv, const char *dir)
     panictrace_setsignals(TRUE);
 #endif
 #endif
+    (void) whoami(); /* set up default plname[] */
 
     prscore(argc, argv);