Add plumbing for ASPA support. This implements the parser and part of the

Add plumbing for ASPA support. This implements the parser and part of the
logic in the rtr process. It does not implement the new RTR messages yet
but it is possible to specify an aspa-set in the config. Also the validation
code in the RDE is missing so this does not do anything.
With this in it will be possible to extend rpki-client to publish an
aspa-set as part of the openbgpd config file.
OK tb@