fixes #143 Protocols and transports should be "configurable"

This makes all the protocols and transports optional.  All
of them except ZeroTier are enabled by default, but you can
now disable them (remove from the build) with cmake options.

The test suite is modified so that tests still run as much
as they can, but skip over things caused by missing functionality
from the library (due to configuration).

Further, the constant definitions and prototypes for functions
that are specific to transports or protocols are moved into
appropriate headers, which should be included directly by
applications wishing to use these.

We have also added and improved documentation -- all of the
transports are documented, and several more man pages for
protocols have been added.  (Req/Rep and Surveyor are still
missing.)

1 files changed, 93 insertions, 0 deletions

diff --git a/docs/nng_push.adoc b/docs/nng_push.adoc
new file mode 100644
index 00000000..faf783b6
--- /dev/null
+++ b/docs/nng_push.adoc
@@ -0,0 +1,93 @@
+nng_push(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_push - push protocol
+
+SYNOPSIS
+--------
+
+[source,c]
+----------
+#include <nng/protocol/pipeline0/push.h>
+
+int nng_push0_open(nng_socket *s);
+----------
+
+DESCRIPTION
+-----------
+
+The _nng_push_ protocol is one half of a pipeline pattern.  The
+other side is the <<nng_pull.adoc#,nng_pull(7)>> protocol.
+
+In the pipeline pattern, pushers distribute messages to pullers. 
+Each message sent
+by a pusher will be sent to one of its peer pullers,
+chosen in a round-robin fashion
+from the set of connected peers available for receiving.
+This property makes this pattern useful in load-balancing scenarios.
+
+Socket Operations
+~~~~~~~~~~~~~~~~~
+
+The `nng_push0_open()` call creates a pusher socket.  This socket
+may be used to send messages, but is unable to receive them.  Attempts
+to receive messages will result in `NNG_ENOTSUP`.
+
+Send operations will observe flow control (back-pressure), so that
+only peers capable of accepting a message will be considered.  If no
+peer is available to receive a message, then the send operation will
+wait until one is available, or the operation times out.
+
+NOTE: Although the pipeline protocol honors flow control, and attempts
+to avoid dropping messages, no guarantee of delivery is made.  Furthermore,
+as there is no capability for message acknowledgement, applications that
+need reliable delivery are encouraged to consider the
+<<nng_req.adoc#,nng_req(7)>> protocol instead.
+
+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 _nng_push_ protocol has no protocol-specific options.
+
+Protocol Headers
+~~~~~~~~~~~~~~~~
+
+The _nng_push_ protocol has no protocol-specific headers.
+    
+AUTHORS
+-------
+link:mailto:garrett@damore.org[Garrett D'Amore]
+
+SEE ALSO
+--------
+<<nng.adoc#,nng(7)>>
+<<nng_pull.adoc#,nng_pull(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].