add MAXPLAYERS to SYSCF (trunk only)

Add MAXPLAYERS to SYSCF config file; deprecate (but continue to support)
 MAX_NR_OF_PLAYERS in nethack.sh since it is trivially overridden in many
 (all?) cases and isn't useful for ports that don't use nethack.sh.

include/sys.h

@@ -13,6 +13,7 @@ struct sysopt {
 	char *support;	/* local support contact */
 	char *recover;	/* how to run recover - may be overridden by win port */
 	char *wizards;
+	int   maxplayers;
 };
 E struct sysopt sysopt;