Use a proper union in struct config_item.

This allows me to use different types for the mutable, dynamically
allocated string value in a C_STRING control and the fixed constant
list of option names in a C_CHOICES.

misc.c

@@ -17,7 +17,7 @@ void free_cfg(config_item *cfg)
 
     for (i = cfg; i->type != C_END; i++)
 	if (i->type == C_STRING)
-	    sfree(i->sval);
+	    sfree(i->u.string.sval);
     sfree(cfg);
 }