rework checksum/vlan offloading, and enable it for ipv6 too.

rework checksum/vlan offloading, and enable it for ipv6 too.

this is based on work by jan@ and bluhm@. the most interesting
change on top of their work is to move the parsing of the IP packets
to figure out ip and udp/tcp offsets before a possible call to
m_defrag.

if an l4 offload is requested, it's only requested by the stack
when the payload is correctly aligned and with each header contiguous
in memory. this means you can use m_getptr and cast the packet data
to the relevant headers to read them directly because that's what
the stack does when it's working on them. this makes it cheap to
work on them too.

however, if you m_defrag, it ignores the alignment and ends up
making it unsafe to dereference the ip and tcp/udp payloads on
strict alignment architectures. if we want to look at the headers
after m_defrag, we'd likely have to copy them onto the stack first.

we got away with this before because we were lucky with the code
that's generated on strict alignment archs for looking at the ip
hl and proto fields. that luck might run out though.

while here i've also tweaked the vlan handling code, and generally
tried to make it more readable.

ok bluhm@ jmatthew@