Fix problems with handling of oversize BGP messagesex24b

The objective is to (as gracefully as possible) deal with outgoing UPDATE
messages which simply do not in the maximum size BGP message.  This can
happen if very large AS_PATH or Community attributes come in, and are
then extended.  (Sending an UPDATE to an AS2 speaking peer can create
a complete second copy of the AS_PATH !)  Previous versions of Quagga
would crash under these circumstances.

Recent version were changed to tolerate and detect oversize messages.
Any oversize message is logged as an error and discarded.

This version will also withdraw prefixes if the selected route simply
cannot be advertised.

This version also corrects the handling of messages which are limited to
the maximum BGP message size -- in particular UPDATE messages which
carry a large number of IPv4/Unicast prefixes.  (This was broken by the
earlier efforts to handle oversize messages.)

Changes in this commit:

  * update version to 0.99.20ex24b

  * where a set of attributes is too big to fit into a BGP message, or
    so big that not even one prefix will also fit into a BGP message,
    the following steps are now taken:

      - the BGP message is discarded and an error logged (as in other
        recent versions -- Quagga does not crash).

      - any prefixes which should have been advertised with the (broken)
        attributes will be withdrawn, if they have been advertised earlier
        with valid attributes.

        A further error is logged, listing the affected prefixes.

      - prefixes which are suppressed in this way are not counted as
        having been sent.

    At present there is no "show" command which will show which prefixes have
    been suppress -- TBA.

  * for all AFI/SAFI announce as many withdrawn prefixes as will fit in a
    BGP message.

    Previously, all AFI/SAFI other than IPv4/Unicast would send one withdrawn
    prefix per message.

    (This is still the case for announcements.  It seems unlikely that
     many IPv6 prefixes will have the same attributes... so this does not
     seem worth fixing immediately.)

  * ensure that NOTIFICATION message cannot exceed the maximum length of a
    BGP message, no matter how much data is sent (!).

  * where IPv4/Unicast prefixes share the same attributes, they are now
    announced in the order received.

    Previously, when prefixes were added to the list hung off the
    'struct bgp_advertise_attr', they were in LIFO order.

  * add SAFI name table.

  * log pthread attributes when creating a pthread.

  * fix scheduling of withdrawn prefixes to reduce number of BGP messages
    generated.

Bug fixes:

  * fix discard of invalid IPv6 link-local nexthop (recent bug).

  * where amount of information that is put into a BGP message is limited by
    the maximum message size, fix so that this works again.  (This was
    broken recently.)

  * set "Bottom of Stack" bit in MPLS VPN "tag" in MP_REACH and MP_UNREACH
    outgoing attributes.

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