fexc: Clarify intent of opt_string adjustment

This also avoids a possible compiler warning (-Wstring-plus-int).
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

fexc.c

@@ -247,7 +247,8 @@ int main(int argc, char *argv[])
 
 	int app_mode = app_choose_mode(argv[0]);
 
-	const char *opt_string = "I:O:vq?"+ ((app_mode == 0)? 0: 4);
+	const char *opt_string = "I:O:vq?";
+	if (app_mode != 0) opt_string += 4; /* disallow -I and -O */
 	int opt, ret = 1;
 	int verbose = 0;