allow wccp processing to be enabled per interface with the link0 flag.

allow wccp processing to be enabled per interface with the link0 flag.

this also changes the wccp handling to peek into it's payload to
determine whether it is wccp 1 or 2. wccp1 says the gre header is
followed by ipv4, while wccp2 says there's a small header before
the ipv4 packet. the wccp2 header cannot have 4 in the first nibble,
while ipv4 must have 4 in the first nibble. the code now looks at
the nibble to determine whether it should strip the wccp2 header
or not.