As part of normal HTTP/2 processing, incomplete frames are saved in the
control state using a fixed size memcpy of NGX_HTTP_V2_STATE_BUFFER_SIZE.
For this matter, two state buffers are reserved in the HTTP/2 recv buffer.

As part of HTTP/2 auto-detection on plain TCP connections, initial data
is first read into a buffer specified by the client_header_buffer_size
directive that doesn't have state reservation.  Previously, this made it
possible to over-read the buffer as part of saving the state.

The fix is to read the available buffer size rather than a fixed size.
Although memcpy of a fixed size can produce a better optimized code,
handling of incomplete frames isn't a common execution path, so it was
sacrificed for the sake of simplicity of the fix.

To ensure that attempts to flood servers with many streams are detected
early, a limit of no more than 2 * max_concurrent_streams new streams per one
event loop iteration was introduced.  This limit is applied even if
max_concurrent_streams is not yet reached - for example, if corresponding
streams are handled synchronously or reset.

Further, refused streams are now limited to maximum of max_concurrent_streams
and 100, similarly to priority_limit initial value, providing some tolerance
to clients trying to open several streams at the connection start, yet
low tolerance to flooding attempts.

Although it has better implementation status than HTTP/3 server push,
it remains of limited use, with adoption numbers seen as negligible.
Per IETF 102 materials, server push was used only in 0.04% of sessions.
It was considered to be "difficult to use effectively" in RFC 9113.
Its use is further limited by badly matching to fetch/cache/connection
models in browsers, see related discussions linked from [1].

Server push was disabled in Chrome 106 [2].

The http2_push, http2_push_preload, and http2_max_concurrent_pushes
directives are made obsolete.  In particular, this essentially reverts
7201:641306096f5b and 7207:3d2b0b02bd3d.

[1] https://jakearchibald.com/2017/h2-push-tougher-than-i-thought/
[2] https://chromestatus.com/feature/6302414934114304

The directive enables HTTP/2 in the current server.  The previous way to
enable HTTP/2 via "listen ... http2" is now deprecated.  The new approach
allows to share HTTP/2 and HTTP/0.9-1.1 on the same port.

For SSL connections, HTTP/2 is now selected by ALPN callback based on whether
the protocol is enabled in the virtual server chosen by SNI.  This however only
works since OpenSSL 1.0.2h, where ALPN callback is invoked after SNI callback.
For older versions of OpenSSL, HTTP/2 is enabled based on the default virtual
server configuration.

For plain TCP connections, HTTP/2 is now auto-detected by HTTP/2 preface, if
HTTP/2 is enabled in the default virtual server.  If preface is not matched,
HTTP/0.9-1.1 is assumed.

Similarly to 7192:d5a535774861, this avoids spurious zero statuses
in access.log, and in line with other header-related errors.

Similarly to ticket #274 (7354:1812f1d79d84), early request finalization
without calling ngx_http_run_posted_requests() resulted in a connection
hang (a socket leak) if the 400 (Bad Request) error was generated in
ngx_http_v2_state_process_header() due to invalid request headers and
"return 444" was used in error_page 400.

Multi headers are now using linked lists instead of arrays.  Notably,
the following fields were changed: r->headers_in.cookies (renamed
to r->headers_in.cookie), r->headers_in.x_forwarded_for,
r->headers_out.cache_control, r->headers_out.link, u->headers_in.cache_control
u->headers_in.cookies (renamed to u->headers_in.set_cookie).

The r->headers_in.cookies and u->headers_in.cookies fields were renamed
to r->headers_in.cookie and u->headers_in.set_cookie to match header names.

The ngx_http_parse_multi_header_lines() and ngx_http_parse_set_cookie_lines()
functions were changed accordingly.

With this change, multi headers are now essentially equivalent to normal
headers, and following changes will further make them equivalent.

With large http2_max_concurrent_streams or http2_max_concurrent_pushes, more
than 255 ngx_http_v2_node_t structures might be allocated, eventually leading
to h2c->closed_nodes overflow when closing corresponding streams.  This will
in turn result in additional allocations in ngx_http_v2_get_node_by_id().

While mostly harmless, it can result in excessive memory usage by a HTTP/2
connection, notably in configurations with many keepalive_requests allowed.
Fix is to use ngx_uint_t for h2c->closed_nodes instead of unsigned:8.

Response headers can be buffered in the SSL buffer.  But stream's fake
connection buffered flag did not reflect this, so any attempts to flush
the buffer without sending additional data were stopped by the write filter.

It does not seem to be possible to reflect this in fc->buffered though, as
we never known if main connection's c->buffered corresponds to the particular
stream or not.  As such, fc->buffered might prevent request finalization
due to sending data on some other stream.

Fix is to implement handling of flush buffers when the c->need_flush_buf
flag is set, similarly to the existing last buffer handling.  The same
flag is now used for UDP sockets in the stream module instead of explicit
checking of c->type.

ngx_http_v2_huff_decode.c and ngx_http_v2_huff_encode.c are renamed
to ngx_http_huff_decode.c and ngx_http_huff_encode.c.