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()

apps/backend/backend_socket.c

@@ -249,7 +249,6 @@ backend_accept_client(int   fd,
     }
     if ((ce = backend_client_add(h, &from)) == NULL)
         goto done;
-    ce->ce_handle = h;
 
     /* 
      * Get credentials of connected peer - only for unix socket