Clean up network header stuff and make it more generic. Use

Clean up network header stuff and make it more generic.  Use
libdnet when we can instead of our own home grown stuff.

SConscript:
    separate the crc code into its own file
base/inet.cc:
    move the crc stuff to crc.cc
    add generic code for calculating ip/tcp/udp checksums
base/inet.hh:
    -  move crc stuff to crc.hh
    -  #include all of the libdnet stuff.  (this makes base/inet.hh the
    only file you need to include if you want to use this kind of stuff.)
    -  Wrap some of the libdnet structs to get easier access to structure
    members.  These wrappers will automatically deal with masking/shifting/
    byte-swapping.
base/refcnt.hh:
    If one derives from RefCountingPtr, they should have access to
    the internal data pointer.
build/SConstruct:
    make #include of dnet stuff work
dev/etherlink.cc:
dev/ethertap.cc:
dev/ethertap.hh:
    EtherPacket -> PacketData
dev/etherpkt.cc:
    EtherPacket -> PacketData
    add a function for populating extra info about a packet.
    Basically just gives pointers to ethernet/ip/tcp/udp headers
    if they exist.
dev/etherpkt.hh:
    EtherPacket -> PacketData
    remove most of the packet header stuff from teh PacketData
    class and just add a few generic functions for grabbing various
    headers that may exist in the packet.  The old functionality is
    contained in the headers.
dev/ns_gige.cc:
    -  IP -> Ip, UDP -> Udp, TCP ->Tcp when used in variable names
    -  get rid of our own byte swapping functions.
    -  whack checksum code and use libdnet version.
    -  Get pointers to the various packet headers and grab info from
    those headers.  (The byte swapping in the headers now.)
    -  Add stats for Udp Checksums
dev/ns_gige.hh:
    use libdnet for checksum code.
    IP -> Ip, TCP -> Tcp in variable names
    add stats for UDP checksums

--HG--
extra : convert_revision : 96c4160e1967b7c0090acd456df4a76e1f3aab53