definitions as aliases, to be removed in release 3.0.  Also renamed the source
files implementing these two socket types.  This change does not break existing
applications nor bindings, but allows us to fix the documentation and user guide
now, rather than keeping the old (confusing) names.

This reverts part of commit 84e0c799 to get
correct ZMQ_HWM semantics with XREQ sockets:

When sending a message to an XREQ socket, the underlying pipe is selected in
a round-robin fashion. If an underlying pipe is full it is skipped. If there
are no underlying pipes, or all underlying pipes are full then zmq_send()
shall block or return EAGAIN, depending on whether or not the call is blocking.
Messages are never dropped.

The msgs_written variable keeps track how many complete
messages have been written so far. The rollback operation drops all
fragments of the last incomplete message so it shouldn't
change this variable at all.