The gRPC protocol makes a distinction between HEADERS frame with
the END_STREAM flag set, and a HEADERS frame followed by an empty
DATA frame with the END_STREAM flag.  The latter is not permitted,
and results in errors not being propagated through nginx.  Instead,
gRPC clients complain that "server closed the stream without sending
trailers" (seen in grpc-go) or "13: Received RST_STREAM with error
code 2" (seen in grpc-c).

To fix this, nginx now returns HEADERS with the END_STREAM flag if
the response length is known to be 0, and we are not expecting
any trailer headers to be added.  And the response length is
explicitly set to 0 in the gRPC proxy if we see initial HEADERS frame
with the END_STREAM flag set.

Unified the style of validity checks in ngx_http_v2_validate_header().

There is no need to calculate hashes of static strings at runtime.  The
ngx_hash() macro can be used to do it during compilation instead, similarly
to how it is done in ngx_http_proxy_module.c for "Server" and "Date" headers.

In particular, if a stream object allocation failed, and a client sent
the PRIORITY frame for this stream, ngx_http_v2_set_dependency() could
dereference a null pointer while trying to re-parent a dependency node.

The Accept-Encoding, Accept-Language, and User-Agent header fields
are now copied from the original request to pushed requests.

r->headers_in.host can be NULL in ngx_http_v2_push_resource().

This happens when a request is terminated with 400 before the :authority
or Host header is parsed, and either pushing is enabled on the server{}
level or error_page 400 redirects to a location with pushes configured.

Found by Coverity (CID 1429156).