Translate the `data->set.httpwant` which is one of the consts from the
public API (CURL_HTTP_VERSION_*) into a major version mask plus
additional flags for internal handling.
`Curl_http_neg_init()` does the translation and flags setting in http.c,
using new internal consts CURL_HTTP_V1x, CURL_HTTP_V2x and CURL_HTTP_V3x
for the major versions. The flags are
- only_10: when the application explicity asked fro HTTP/1.0
- h2_upgrade: when the application asks for upgrading 1.1 to 2.
- h2_prior_knowledge: when directly talking h2 without ALPN
- accept_09: when a HTTP/0.9 response is acceptable.
The Alt-Svc and HTTPS RR redirections from one ALPN to another obey the
allowed major versions. If a transfer has only h3 enabled, Alt-Svc
redirection to h2 is ignored.
This is the current implementation. It can be debated if Alt-Svc should
be able to override the allowed major versions. Added test_12_06 to
verify the current restriction.
Closes#16100
db72b8d4d0