Various FRR enhancements
Started off tidying up the BFD integrating in #835 and found a few other things to tidy up.
1. Extend #10441 to better integrate with the current FRR configuration style and add support for BFD to BGP, OSPF and OSPFv3 daemons.
2. Make the OSPF and OSPFv3 configuration flow more alike.
3. Allow the OSPF/OSPFv3 default area to be unspecified as long as the area is specified on the interface for situations where a default is not preferable.
3. Add a check to prevent a situation where BGP can be started with no local AS set.
4. Tidy up the configuration by only generating ACCEPTFILTER when there the convenience filter functions in the UI have been used.
5. Add agentx support to FRR to allow SNMP data retrieval when used with the net-snmp package.
Tested on 2.4.5 release with FRR 7.2.1 and 7.3.