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_android.go

@@ -0,0 +1,25 @@
+package net
+
+import (
+	"syscall"
+
+	log "github.com/sirupsen/logrus"
+)
+
+func (d *Dialer) init() {
+	d.Dialer.Control = func(_, _ string, c syscall.RawConn) error {
+		err := c.Control(func(fd uintptr) {
+			androidProtectSocketLock.Lock()
+			f := androidProtectSocket
+			androidProtectSocketLock.Unlock()
+			if f == nil {
+				return
+			}
+			ok := f(int32(fd))
+			if !ok {
+				log.Errorf("failed to protect socket: %d", fd)
+			}
+		})
+		return err
+	}
+}