Vendor identifying vendor things

[reynir]

This implements https://www.rfc-editor.org/rfc/rfc3925.html except Vi_vendor_info leaves the sub options unparsed (they must be dhcp-style TLVs; this implementation leave them as an opaque string). I may implement that as well.

I wrote a test from my understanding of the option (and an incompatible sample). To make the test work better I had to change the semantics of options_of_buf so that it is not responsible for skipping the BOOTP packet. This is also more in line with the behavior of buf_of_options. This is a breaking change, but I have found no outside users of that underdocumented function. The test is very synthetic, and more tests would be nice. I haven't otherwise tested it.