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/http/test_12_reuse.py

@@ -98,6 +98,7 @@ class TestReuse:
         for s in r.stats:
             assert s['http_version'] == '3', f'{s}'
 
+    @pytest.mark.skipif(condition=not Env.have_h3(), reason="h3 not supported")
     def test_12_04_alt_svc_h3h2(self, env: Env, httpd, nghttpx):
         httpd.clear_extra_configs()
         httpd.reload()
@@ -115,11 +116,12 @@ class TestReuse:
             '--alt-svc', f'{asfile}',
         ])
         r.check_response(count=count, http_status=200)
-        # We expect the connection to be reused
+        # We expect the connection to be reused and use HTTP/2
         assert r.total_connects == 1
         for s in r.stats:
             assert s['http_version'] == '2', f'{s}'
 
+    @pytest.mark.skipif(condition=not Env.have_h3(), reason="h3 not supported")
     def test_12_05_alt_svc_h3h1(self, env: Env, httpd, nghttpx):
         httpd.clear_extra_configs()
         httpd.reload()
@@ -137,9 +139,7 @@ class TestReuse:
             '--alt-svc', f'{asfile}',
         ])
         r.check_response(count=count, http_status=200)
-        # We expect the connection to be reused
+        # We expect the connection to be reused and use HTTP/1.1
         assert r.total_connects == 1
-        # When using http/1.1 from alt-svc, we ALPN-negotiate 'h2,http/1.1' anyway
-        # which means our server gives us h2
         for s in r.stats:
-            assert s['http_version'] == '2', f'{s}'
+            assert s['http_version'] == '1.1', f'{s}'