fixes #148 man pages for req/rep protocol needed

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+nng_rep(7)
+==========
+:doctype: manpage
+:manmanual: nng
+:mansource: nng
+:icons: font
+:source-highlighter: pygments
+:copyright: Copyright 2017 Garrett D'Amore <garrett@damore.org> \
+            Copyright 2017 Capitar IT Group BV <info@capitar.com> \
+            This software is supplied under the terms of the MIT License, a \
+            copy of which should be located in the distribution where this \
+            file was obtained (LICENSE.txt).  A copy of the license may also \
+            be found online at https://opensource.org/licenses/MIT.
+
+NAME
+----
+nng_rep - reply protocol
+
+SYNOPSIS
+--------
+
+[source,c]
+----------
+#include <nng/protocol/reqrep0/rep.h>
+
+int nng_rep0_open(nng_socket *s);
+----------
+
+DESCRIPTION
+-----------
+
+The _nng_rep_ protocol is one half of a request/reply pattern.
+In this pattern, a requester sends a message to one replier, who
+is expected to reply.  The request is resent if no reply arrives,
+until a reply is received or the request times out.
+
+TIP: This protocol is useful in setting up RPC-like services.  It
+is also "reliable", in that a the requester will keep retrying until
+a reply is received.
+
+The _nng_rep_ protocol is the replier side, and the
+<<nng_req.adoc#,nng_req(7)>> protocol is the requester side.
+
+Socket Operations
+~~~~~~~~~~~~~~~~~
+
+The `nng_rep0_open()` call creates a requester socket.  This socket
+may be used to receive messages (requests), and then to send replies.  Generally
+a reply can only be sent after receiving a request.  (Attempts to receive
+a message will result in `NNG_ESTATE` if there is no outstanding request.)
+
+Attempts to send on a socket with no outstanding requests will result
+in `NNG_ESTATE`. 
+
+Raw mode sockets (set with `NNG_OPT_RAW`) ignore all these restrictions.
+
+Protocol Versions
+~~~~~~~~~~~~~~~~~
+
+Only version 0 of this protocol is supported.  (At the time of writing,
+no other versions of this protocol have been defined.)
+
+Protocol Options
+~~~~~~~~~~~~~~~~
+
+The following protocol-specific options are available.
+
+`NNG_OPT_MAXTTL`::
+
+   Maximum time-to-live.  This option is an integer value
+   between 0 and 255,
+   inclusive, and is the maximum number of "hops" that a message may
+   pass through until it is discarded.  The default value is 8.  A value
+   of 0 may be used to disable the loop protection, allowing an infinite
+   number of hops.
+
+Protocol Headers
+~~~~~~~~~~~~~~~~
+
+The _nng_rep_ protocol uses a _backtrace_ in the header.  This
+form uses an array of 32-bit big-endian identifiers, where the first
+element in the array
+identifies the local peer identifier to which the message will next be sent.
+This is a hop-by-hop header where each element in a path adds routing
+information to the end when sending a request, and when replying removes
+elements to obtain the next hop information.  The request ID is at the
+end of this header and is inserted into the header as its first element
+by the originating surveyor.  (Request IDs are distinguished from hops by
+having their high order bit set to one.  They are generated automatically
+and randomly when a request is first issued.)
+
+// TODO: Insert reference to RFC.
+
+    
+AUTHORS
+-------
+link:mailto:garrett@damore.org[Garrett D'Amore]
+
+SEE ALSO
+--------
+<<nng.adoc#,nng(7)>>
+<<nng_req.adoc#,nng_req(7)>>
+
+COPYRIGHT
+---------
+
+Copyright 2017 mailto:garrett@damore.org[Garrett D'Amore] +
+Copyright 2017 mailto:info@capitar.com[Capitar IT Group BV]
+
+This document is supplied under the terms of the
+https://opensource.org/licenses/LICENSE.txt[MIT License].