Problem: Zproxy does not do anything

We could create a new Zproxy and destroy it, but that was it.  Added:

* Zproxy.SetFrontend - sends a command to the czmq zproxy to create a new frontend socket
* Zproxy.SetBackend - sends a command to the czmq zproxy to create a new backend socket
* Zproxy.SetCapture - sends a command to the czmq zproxy to create a new capture socket

Note - cgo does not support variadic C functions.  Originally to get around this limitation I wrote a shim C function that wrapped zstr_sendm for sending proxy commands.  To remove the dependency on that shim, I changed the Zproxy commands to use C.zstr_sendm to send the commands as multi part messages.