Feature/exit node Android (#1916)

Support exit node on Android.
With the protect socket function, we mark every connection that should be used out of VPN.

util/net/dialer_ios.go

@@ -0,0 +1,13 @@
+package net
+
+import (
+	"net"
+)
+
+func DialUDP(network string, laddr, raddr *net.UDPAddr) (*net.UDPConn, error) {
+	return net.DialUDP(network, laddr, raddr)
+}
+
+func DialTCP(network string, laddr, raddr *net.TCPAddr) (*net.TCPConn, error) {
+	return net.DialTCP(network, laddr, raddr)
+}