a bit more cleaning of the network header wrappers.

a bit more cleaning of the network header wrappers.

base/inet.hh:
    add functions to the various headers to grab the most common
    encapsulated protocols.  This could easily get out of hand, but
    we're just worrying about tcp, udp, and ip for now.

    add common functions size(), bytes(), and payload() to all wrappers.
    size() gets the header size
    bytes() returns a uint8_t * to the beginning of the header
    payload() returns a uint8_t * to the beginning of the payload.
dev/etherpkt.cc:
dev/etherpkt.hh:
    don't cache pointers to headers.  It's probably not worth the
    hassle.

--HG--
extra : convert_revision : ba9df85ac019b8a48233042dde79fb9da9546410