Add socket option BINDTODEVICE

Linux now supports Virtual Routing and Forwarding (VRF) as per:

https://www.kernel.org/doc/Documentation/networking/vrf.txt

In order for an application to bind or connect to a socket with an
address in a VRF, they need to first bind the socket to the VRF device:

    setsockopt(sd, SOL_SOCKET, SO_BINDTODEVICE, dev, strlen(dev)+1);

Note "dev" is the VRF device, eg. VRF "blue", rather than an interface
enslaved to the VRF.

Add a new socket option, ZMQ_BINDTODEVICE, to bind a socket to a device.
In general, if a socket is bound to a device, eg. an interface, only
packets received from that particular device are processed by the socket.

If device is a VRF device, then subsequent binds/connects to that socket
use addresses in the VRF routing table.

src/tcp_connecter.cpp

@@ -305,6 +305,10 @@ int zmq::tcp_connecter_t::open ()
     if (options.tos != 0)
         set_ip_type_of_service (s, options.tos);
 
+    // Bind the socket to a device if applicable
+    if (!options.bound_device.empty ())
+        bind_to_device (s, options.bound_device);
+
     // Set the socket to non-blocking mode so that we get async connect().
     unblock_socket (s);