fix: 'nbr ... fall-over' bfd/route-map coexist

previously, 'neighbor ... fall-over' with BFD and route-map options
were treated as mutually exclusive and the use of one replaced the
other - this is not the case and both can exist independently

in addition, 'neighbor ... fall-over' (without either option) is
really a particular case of the route-map version (matching if there is
no route to the neighbor, rather than a change related to a specific
route-map) - this fixes that

note that there is still a bug here - if the plain form ('neighbor ...
fall-over') is removed, this will cancel the BFD configuration: there
is no way to actually do this in IOS, so it is necessary to clear it
and then restore the BFD configuration - this needs to be fixed,
probably with a trigger