Expose a property to set Dialer.Control

[danielealbano]

The PR introduces a DialerControl property to allow a caller to execute syscalls on the socket before a connection is established.

Binding an IP via Source property is not always an option, in my case I am using a series of devices which get all the same IP and all have the same subnet: the only way to control which one has to be used is to bind the socket to a specific interface.

Here an example of the code

interfaceName := "XYZ"
speedTestClient = speedtest.New(speedtest.WithUserConfig(&speedtest.UserConfig{
	DialerControl: func(network, address string, conn syscall.RawConn) error {
		var operr error
		if err = conn.Control(func(fd uintptr) {
			operr = syscall.SetsockoptString(int(fd), syscall.SOL_SOCKET, syscall.SO_BINDTODEVICE, interfaceName)
		}); err != nil {
			return err
		}

		if operr != nil {
			return operr
		}

		return nil
	},
}))

This allows speedtest-go to work directly on the interface without having to bind on a specific IP.

The PR doesn't change the dialer used for the resolver as the library alters the DefaultResolver.

NOTE: SO_BINDTODEVICE is Linux specific, not sure what the other OS support, but the PR has no impact on the portability of the library itself as it only allows to define the Control property of the Dialer

[r3inbowari]

Thank you!