rewrite dhcpv6 parsing so it follows the rfc, not an incompat draft.

it looks like this code was using draft-ietf-dhc-dhcpv6-14 from
1999. there were 27 drafts, and by the time it got to draft 23 and
the rfc it was significantly different. this code for draft 14
cannot handle actual dhcpv6 messages. im kind of surprised
(disappointed?) that noone noticed before. i only noticed cos the
code was segfaulting on sparc64, and when i tried to fix it the
resulting messages looked nothing like what stock tcpdump produced.

the main difference between the early drafts and what ended up in
the rfc is that the base dhcpv6 messages in early drafts were large
structure with a lot of fixed fields, while the rfc settled on a 4
byte header that contains a 1 byte message type and a 3 byte
transaction id. the rest of the messages are built from dhcp options
fields.

this cuts all the old handling out, and fixes the fault in the
options handling by using EXTRACT_16BITS to get at the code and
length fields instead of using ntohs. dhcpv6 explicitly states that
it does not align options, so this is necessary to avoid faults on
strict alignment archs anyway. no options are pretty printed at the
moment, you just get a numeric type, a length, and a hexdump of the
value. this is still better than the garbage that the draft parsing
produced.

if someone is interested in making this easier to read, it would
be a straightforward and  well contained project to better handle
option printing.

ok deraadt@

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