HTTP/2: reject requests with out-of-order pseudo-headers

Handling of pseudo-headers is refactored to be more in line with
HTTP/3 implementation, that is, a request line is now constructed
as soon as pseudo-headers are followed by regular fields.  Here
this plugs a missing handling for absent mandatory or out-of-order
pseudo-headers.

Such requests are now rejected immediately as malformed.

Closes: https://github.com/nginx/nginx/issues/1526