Add CHECK_PLNAME to sysconf

Setting CHECK_PLNAME to 1 makes WIZARDS, EXPLORERS, and SHELLERS
check the player name instead of the user's login name.

This is mostly useful for public servers which have external
login system and don't create user accounts for players.

include/sys.h

@@ -22,6 +22,7 @@ struct sysopt {
     int maxplayers;
     int seduce;
     int check_save_uid; /* restoring savefile checks UID? */
+    int check_plname; /* use plname for checking wizards/explorers/shellers */
 
     /* record file */
     int persmax;