Netconf monitoring RFC 6022 Sessions (https://github.com/clicon/clixon/issues/370)

- statistics and transport/source-host parameters
  - extended internal NETCONF hello with transport and source-host attributes
clixon-lib,yang
  - Moved all extended internal NETCONF attributes to the clicon-lib namespace
C-API:
  - wrapped most attribute creation into new fn xml_add_attr()

lib/src/clixon_xml_nsctx.c

@@ -569,6 +569,7 @@ xml2ns_recurse(cxobj *xt)
  * @retval     0          OK
  * @retval    -1          Error
  * @see xml2ns 
+ * @see xml_add_attr  generic method for adding an attribute
  */
 int
 xmlns_set(cxobj *x,