lib: clarify 'conn->httpversion'

The variable `conn->httpversion` was used for several purposes and it
was unclear at which time the value represents what.

- rename `conn->httpversion` to `conn->httpversion_seen`
  This makes clear that the variable only records the last
  HTTP version seen on the connection - if any. And that it
  no longer is an indication of what version to use.
- Change Alt-Svc handling to no longer modify `conn->httpversion`
  but set `data->state.httpwant` for influencing the HTTP version
  to use on a transfer.
- Add `data->req.httpversion_sent` to have a record of what
  HTTP version was sent in a request
- Add connection filter type CF_TYPE_HTTP
- Add filter query `CF_QUERY_HTTP_VERSION` to ask what HTTP
  filter version is in place
- Lookup filters HTTP version instead of using `conn->httpversion`

Test test_12_05 now switches to HTTP/1.1 correctly and the
expectations have been fixed.

Removed the connection fitler "is_httpN()" checks and using
the version query instead.

Closes #16073

tests/data/test471

@@ -68,7 +68,7 @@ Accept: */*
 </protocol>
 # curl: (1) Version mismatch (from HTTP/1 to HTTP/2)
 <errorcode>
-1
+8
 </errorcode>
 </verify>
 </testcase>