reject chunked transfer-encoding in HTTP/1.0 requests

[sahvx655-wq]

HTTP/1.0 has no chunked transfer coding, yet the parser honoured it on a request and framed the body as chunks. I was tracing how finish_header decides the body style and the version flag sits right next to the framing decision but is never read.

1. do_field sets the chunked flag whenever Transfer-Encoding resolves to chunked as the final coding, with no regard for the request version.
2. finish_header then switches into chunk parsing, so POST / HTTP/1.0 carrying Transfer-Encoding: chunked is consumed as a chunked message.

Reject at finish_header when the chunked flag is set on anything below HTTP/1.1. The risk if left alone is desync: a front-end that follows HTTP/1.0 and ignores Transfer-Encoding will disagree with Beast about where the body ends, which is a request smuggling primitive. Responses are left untouched since reading a 1.0 chunked response to EOF is the usual interop fudge. Closes #837.