vmd(8): pull validation into local prefix parser.

vmd(8): pull validation into local prefix parser.

Validation for local prefixes, both inet and inet6, was scattered
around. To make it even more confusing, vmd was using generic address
parsing logic from prior network daemons. vmd doesn't need to parse
addresses other than when parsing the local prefix settings in
vm.conf and no runtime parsing is needed.

This change merges parsing and validation based on vmd's specific
needs for local prefixes (e.g. reserving enough bits for vm id and
network interface id encoding in an ipv4 address). In addition, it
simplifies the struct from a generic address struct to one focused
on just storing the v4 and v6 prefixes and masks. This cleans up an
unused TAILQ struct member that isn't used by vmd and was leftover
copy-pasta from those prior daemons.

The address parsing that vmd uses is also updated to using the
latest logic in bgpd(8).

ok mlarkin@