Merging versioned files into one, so `go get` will work for all
versions of ZeroMQ without use of -tags, and gozmq will function
identical for all versions of ZeroMQ.

The following stuff is (temporarily) removed:

Zmq 2.1 & 2.2:

        RECOVERY_IVL_MSEC = Int64SocketOption(C.ZMQ_RECOVERY_IVL_MSEC)
        SWAP              = Int64SocketOption(C.ZMQ_SWAP)
        MCAST_LOOP        = Int64SocketOption(C.ZMQ_MCAST_LOOP)
        HWM               = UInt64SocketOption(C.ZMQ_HWM)

Zmq 2.2:

        RCVTIMEO = IntSocketOption(C.ZMQ_RCVTIMEO)
        SNDTIMEO = IntSocketOption(C.ZMQ_SNDTIMEO)

Zmq 3.x:

        SNDHWM = IntSocketOption(C.ZMQ_SNDHWM)
        RCVHWM = IntSocketOption(C.ZMQ_SNDHWM)

        // TODO Not documented in the man page...
        //LAST_ENDPOINT       = UInt64SocketOption(C.ZMQ_LAST_ENDPOINT)
        FAIL_UNROUTABLE     = BoolSocketOption(C.ZMQ_FAIL_UNROUTABLE)
        TCP_KEEPALIVE       = IntSocketOption(C.ZMQ_TCP_KEEPALIVE)
        TCP_KEEPALIVE_CNT   = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_CNT)
        TCP_KEEPALIVE_IDLE  = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_IDLE)
        TCP_KEEPALIVE_INTVL = IntSocketOption(C.ZMQ_TCP_KEEPALIVE_INTVL)
        // TODO Make this work.
        //TCP_ACCEPT_FILTER   = IntSocketOption(C.ZMQ_TCP_ACCEPT_FILTER)

        // Message options
        MORE = MessageOption(C.ZMQ_MORE)