Problem: client send API is too clumsy for common use

Having to send zmsg_t structures is fine for real applications
yet it's clumsy for simpler cases.

Solution: provide additional API that focusses on string data.

    mlm_client_sendx (send string data to stream)
    mlm_client_sendto (send string data to mailbox)
    mlm_client_sendfor (send string data to service)

These take multiple strings, sending as multiframe data. You end
the argument list with NULL (same style as zstr_sendx, where this
is taken from).