fix some bugs in the headers and fix checksumming.

fix some bugs in the headers and fix checksumming.

base/inet.cc:
    we can't use a uint16_t for the intermediate checksum values
    since there may be some bits that carry.
    the length that is added in the pseudo header is the length of
    the tcp portion only.
base/inet.hh:
    silly me, the ip_hlen field is in terms of 4-byte words.
    when caclulating checksum, we're dealing with network byte order,
    so don't bother doing any byteswapping.

--HG--
extra : convert_revision : 993e3413e9febea0ba2fb6ba8bf04c053cca15ed