RFC 1048 and 1534 allow us to put DHCP stuff into the BOOTP vendor section.

This still doesn't make the unknown switch in our network to get working,
but at least it looks consistent with RFCs and pretty in Wireshark.

We have to promise the 'vend' field will be always exactly 64 bytes, though.

Signed-off-by: Martin Pelikan <pelikan@storkhole.cz>