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/clixon/clixon_netconf_monitoring.h

@@ -41,6 +41,6 @@
 /*
  * Prototypes
  */
-int netconf_monitoring_state_get(clicon_handle h, yang_stmt *yspec, char *xpath, cvec *nsc, int brief, cxobj **xret);
+int netconf_monitoring_state_get(clicon_handle h, yang_stmt *yspec, char *xpath, cvec *nsc, cxobj **xret, cxobj **xerr);
 
 #endif  /* _CLIXON_NETCONF_MONITORING_H_ */