fixes #4 Statistics support

This introduces new public APIs for obtaining statistics,
and adds some generic stats for dialers, listeners, pipes, and
sockets.  Also added are stats for inproc and pairv1 protocol.

The other protocols and transports will have stats added
incrementally as time goes on.

A simple test program, and man pages are provided for this.
Start by looking at nng_stat(5).

Statistics does have some impact, and they can be disabled by
using the advanced NNG_ENABLE_STATS (setting it to OFF, it's
ON by default) if you need to build a minimized configuration.

1 files changed, 51 insertions, 0 deletions

diff --git a/docs/man/nng_stat_string.3.adoc b/docs/man/nng_stat_string.3.adoc
new file mode 100644
index 00000000..9f44b5b7
--- /dev/null
+++ b/docs/man/nng_stat_string.3.adoc
@@ -0,0 +1,51 @@
+= nng_version(3)
+//
+// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
+// Copyright 2018 Capitar IT Group BV <info@capitar.com>
+//
+// This document 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_stat_string - get statistic string value
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/nng.h>
+
+typedef struct nng_stat nng_stat;
+
+const char *nng_stat_string(nng_stat *stat);
+----
+
+== DESCRIPTION
+
+The `nng_stat_string()` function returns a string value for the statistic _stat_,
+which must be of type `NNG_STAT_STRING` (see `<<nng_stat_type.3#,nng_stat_type(3)>>`).
+
+If the statistic is not of type `NNG_STAT_STRING`, then `NULL` will be returned.
+
+== RETURN VALUES
+
+The string value associated with _stat_, or `NULL` if the statistic is not
+a string type.
+
+== ERRORS
+
+None.
+
+== SEE ALSO
+
+[.text-left]
+<<libnng.3#,libnng(3)>>,
+<<nng_stats_get.3#,nng_stats_get(3)>>,
+<<nng_stat_type.3#,nng_stats_type(3)>>,
+<<nng_stat_value.3#,nng_stats_value(3)>>,
+<<nng_stat.5#,nng_stat(5)>>,
+<<nng.7#,nng(7)>>