bgpd: fix issue in capability negotiation (BZ#700)

bgpd: fix issue in capability negotiation (BZ#700)

Address problem where bgpd would reject a session if a peer sent some
capabilities in its Open message, but did not include a Multiprotocol
extensions capability. Note that the session would come up if there
were no capabilities at all in the Open message.

  * Add the 'mp_capability' out parameter to
    bgp_capability_parse(). Set it to '1' if a Multiprotocol
    extensions capability is encountered.

  * Switch on 'mp_capability' instead of 'capability' in the calling
    functions to determine if the peer indicated the set of AFI/SAFIs
    it supports.

    The net result is that when a peer does not send an MP capability,
    it is assumed to support the AFI/SAFIs configured for it locally.