* Added Clixon Restconf library

* Builds and installs a new restconf library: libclixon_restconf.so and clixon_restconf.h
  * The restconf library can be included by a restconf plugin.
  * Example code in example/Makefile.in and example/restconf_lib.c
* Authorization
  * Example extended with authorization
  * Test added with http basic authorization (test/test_auth.sh)
  * Documentation in FAQ.md
* README.md extended with new yang, netconf, restconf, datastore, and auth sections.

doc/FAQ.md

@@ -303,4 +303,22 @@ fib_route(clicon_handle h,            /* Clicon handle */
     return 0;
 }
 ```
-Here, the callback is over-simplified.
+Here, the callback is over-simplified.
+
+## How do I write an authentication callback?
+
+A restconf call may need to be authenticated. 
+You can specify an authentication callback for restconf as follows:
+```
+int
+plugin_credentials(clicon_handle h,     
+		   FCGX_Request *r,
+		   char        **username)
+```
+
+If a plugin is provided, it needs to supply a username. If not, the
+request is unauthorized. the function mallocs a username and returns
+it.
+
+See [../apps/example/routing_restconf.c] plugin_credentials() for
+an example of HTTP basic auth.