command line improvement (#115)

make -l easy to use:
-l wg wss
-l wg:12345
-l 12345

make -r use random port

easytier/src/instance/virtual_nic.rs

@@ -58,7 +58,7 @@ impl Stream for TunStream {
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<StreamItem>> {
         let mut self_mut = self.project();
         let mut g = ready!(self_mut.l.poll_lock(cx));
-        reserve_buf(&mut self_mut.cur_buf, 2500, 128 * 1024);
+        reserve_buf(&mut self_mut.cur_buf, 2500, 32 * 1024);
         if self_mut.cur_buf.len() == 0 {
             unsafe {
                 self_mut.cur_buf.set_len(*self_mut.payload_offset);