Implement code to parse, print and reload roa-set tables.

Implement code to parse, print and reload roa-set tables.
This is sharing a lot of code with prefixset which makes all a bit easier.
A roa-set is defined like this:
roa-set "test2" {
  1.2.3.0/24 source-as 1,
  1.2.8.0/22 maxlen 24 source-as 3
}
No support for acting on this data yet.
Put it in deraadt@, OK benno@, input and OK denis@