Add some basic documentation for XREQ/XREP socket types, including
a brief description of the most common use case (REQ -> XREP) and (XREQ ->
REP).

Function returning unsigned long int cannot return (-1)

http://github.com/zeromq/zeromq2/issues#issue/46
Invalid messages are now discarded silently, instead of causing an assertion
failure.

* Fixed zmq_term, zmq_socket, zmq_close, zmq_setsockopt,
* zmq_getsockopt, zmq_bind, zmq_connect, zmq_send,
* zmq_recv, zmq_poll, zmq_device, zmq_stopwatch_stop
* Updated Reference Manual for these methods

* zmq_term
* zmq_socket
* zmq_close
* zmq_setsockopt
* zmq_getsockopt
* zmq_bind
* zmq_connect
* zmq_send
* zmq_recv
* zmq_poll
* zmq_device
* zmq_stopwatch_stop

This patch restarts the send() or recv() inside zmq::signaler_t if the call fails
due to EINTR.

* Clarified broker model and proxy model
* Added example of proxy model

- fixed unwrapped text in new man pages
- fixed over-long lines in older pages, where possible
- removed reference to old standalong devices from index page
- added refernce to new zmq_device[3] documented from index page
- some minor spelling corrections

* Added documentation for zmq_deviced, which we're developing
* Created consistent page footer in documentation template
* Page footer notes doc authors and copyright statement

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.