Add a missing const.

The variable in question holds the return value of strchr when its
input string was const, so it ought logically to be const itself even
though the official prototype of strchr permits it not to be.

config.c

@@ -1227,7 +1227,7 @@ static void portfwd_handler(union control *ctrl, void *dlg,
 		if (key) {
 		    static const char *const afs = "A46";
 		    static const char *const dirs = "LRD";
-		    char *afp;
+		    const char *afp;
 		    int dir;
 #ifndef NO_IPV6
 		    int idx;