set Mode::UdpOnly via command line -u

- fixes #577
- Why the original code use merge instead of replace?

bin/ssmanager.rs

@@ -203,7 +203,7 @@ fn main() {
         // Overrides
         if matches.is_present("UDP_ONLY") {
             if let Some(ref mut m) = config.manager {
-                m.mode = m.mode.merge(Mode::UdpOnly);
+                m.mode = Mode::UdpOnly;
             }
         }
 

bin/ssserver.rs

@@ -178,7 +178,7 @@ fn main() {
             }
 
             if matches.is_present("UDP_ONLY") {
-                sc.set_mode(sc.mode().merge(Mode::UdpOnly));
+                sc.set_mode(Mode::UdpOnly);
             }
 
             if matches.is_present("TCP_AND_UDP") {