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 cache server per protocol.

A compilation has to be hacked with:
  $ ./configure LIBS='-lssh' ...

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

  protocol rpki {
      roa table roatable;
      cache "rpki-validator.realmv6.org";
  }

  protocol rpki {
    roa table roatable;
    cache "localhost" {
      port 2222;
      ssh encryption {
        bird private key "/home/birdgeek/.ssh/id_rsa";
        cache public key "/home/birdgeek/.ssh/known_hosts";
        user "birdgeek";
      };
    };
  }
  ...

TODO list:
 - load libssh2 using dlopen
 - support more cache servers per protocol