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/snmp/README.md

@@ -0,0 +1,32 @@
+SNMP
+====
+
+The SNMP frontend acts as an intermediate daemon between the Net-SNMP
+daemon (snmpd) and the Clixon backend. Clixon-snmp communicates over the AgentX
+protocol to snmpd typically via a UNIX socket, and over the internal IPC protocol to the Clixon
+backend.
+
+Use Net-SNMP version 5.9 or later
+
+To set up AgentX communication between ``clixon_snmp`` and ``snmpd`` a
+Unix or TCP socket is configured. This socket is also configured in
+Clixon (see below). An example `/etc/snmpd/snmpd.conf` is as follows::
+
+   master       agentx
+   agentaddress 127.0.0.1,[::1]
+   rwcommunity  public localhost
+   agentXSocket unix:/var/run/snmp.sock
+   agentxperms  777 777
+
+It is necessary to ensure snmpd does `not` to load modules
+implemented by Clixon. For example, if Clixon implements the IF-MIB and
+system MIBs, snmpd should not load those modules. This can be done
+using the "-I" flag and prepending a "-" before each module::
+   
+   -I -ifTable -I -system_mib -I -sysORTable
+
+Net-snmp must be started via systemd or some other external mechanism before clixon_snmp is started.
+
+To build the snmp support, netsnmp is enabled at configure time.  Two configure  options are added for SNMP:
+* ``--enable-netsnmp`` Enable SNMP support.
+* ``--with-mib-generated-yang-dir`` For tests: Directory of generated YANG specs (default: $prefix/share/mibyang)