Make argument parsing errors fatal

When you run "awesome --foobar", a warning will be printed (by getopt_long())
and awesome just ignores the invalid argument. That's unusual and weird.

This commit produces an error instead.

Signed-off-by: Uli Schlachter <psychon@znc.in>

awesome.c

@@ -524,6 +524,9 @@ main(int argc, char **argv)
           case 'r':
             replace_wm = true;
             break;
+          default:
+            exit_help(EXIT_FAILURE);
+            break;
         }
 
     /* Get XDG basedir data */