make bind resolve the port and fix `this` bug

lib/rpc.js

@@ -51,7 +51,7 @@ module.exports = class RPC {
   bind (port = this._bind) {
     if (this._binding) return this._binding
 
-    const defaultPort = this._bind
+    const self = this
 
     this._binding = new Promise((resolve, reject) => {
       const s = this.socket
@@ -61,16 +61,16 @@ module.exports = class RPC {
       s.on('error', onerror)
 
       function onlistening () {
-        this._bound = true
+        self._bound = true
 
         s.removeListener('listening', onlistening)
         s.removeListener('error', onerror)
-        resolve()
+        resolve(s.address().port)
       }
 
       function onerror (err) {
         // retry on any port if preferred port is unavail
-        if (port === defaultPort && port !== 0) {
+        if (port === self._bind && port !== 0) {
           port = 0
           s.bind(0)
           return