RPKI protocol with integrated RTRLib inside

Add the RPKI protocol (RFC 6810) using the RTRLib
(http://rpki.realmv6.org/) that is integrated inside
the BIRD's code.

Implemeted transports are:
 - unprotected transport over TCP
 - secure transport over SSHv2

The code should work properly with one or more cache servers per protocol.

Example configuration of bird.conf:
  ...
  roa4 table roatable;

  protocol rpki {
    table roatable;

    cache 127.0.0.1; # defaults: port 8282, preference 1, no encryption

    cache 127.0.0.1 {
      preference 1;
      port 2222;
      ssh encryption {
        bird private key "/home/birdgeek/.ssh/id_rsa";
        cache public key "/home/birdgeek/.ssh/known_hosts";
        user "birdgeek";
      };
    };

    cache "rpki-validator.realmv6.org" {
      preference 2;
    };
  }
  ...