Triggered by a crash detected by OSS-Fuzz after the dynbuf introduction in
ed35d6590e72. This should make the trailer handling more straight forward and
hopefully less error-prone.

Deliver the trailer header to the callback already at receive-time. No
longer caches the trailers to get delivered at end of stream.

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=22030
Closes #5348

A common set of functions instead of many separate implementations for
creating buffers that can grow when appending data to them. Existing
functionality has been ported over.

In my early basic testing, the total number of allocations seem at
roughly the same amount as before, possibly a few less.

See docs/DYNBUF.md for a description of the API.

Closes #5300

Prior to this change in libcurl debug builds http2 stream closure was
erroneously referred to as connection closure.

Before:
* nread <= 0, server closed connection, bailing

After:
* nread == 0, stream closed, bailing

Closes https://github.com/curl/curl/pull/5118

Closes #5010

This reduces the HTTP/2 window size to 32 MB since libcurl might have to
buffer up to this amount of data in memory and yet we don't want it set
lower to potentially impact tranfer performance on high speed networks.

Requires nghttp2 commit b3f85e2daa629
(https://github.com/nghttp2/nghttp2/pull/1444) to work properly, to end
up in the next release after 1.40.0.

Fixes #4939
Closes #4940

To simplify our code and since earlier versions lack important function
calls libcurl needs to function correctly.

nghttp2 1.12.0 was relased on June 26, 2016.

Closes #4961

A regression made the code use 'multiplexed' as a boolean instead of the
counter it is intended to be. This made curl try to "over-populate"
connections with new streams.

This regression came with 41fcdf71a1, shipped in curl 7.65.0.

Also, respect the CURLMOPT_MAX_CONCURRENT_STREAMS value in the same
check.

Reported-by: Kunal Ekawde
Fixes #4779
Closes #4784

It could accidentally let the connection get used by more than one
thread, leading to double-free and more.

Reported-by: Christopher Reid
Fixes #4544
Closes #4557

- Disable warning C4127 "conditional expression is constant" globally
  in curl_setup.h for when building with Microsoft's compiler.

This mainly affects building with the Visual Studio project files found
in the projects dir.

Prior to this change the cmake and winbuild build systems already
disabled 4127 globally for when building with Microsoft's compiler.
Also, 4127 was already disabled for all build systems in the limited
circumstance of the WHILE_FALSE macro which disabled the warning
specifically for while(0). This commit removes the WHILE_FALSE macro and
all other cruft in favor of disabling globally in curl_setup.

Background:

We have various macros that cause 0 or 1 to be evaluated, which would
cause warning C4127 in Visual Studio. For example this causes it:

    #define Curl_resolver_asynch() 1

Full behavior is not clearly defined and inconsistent across versions.
However it is documented that since VS 2015 Update 3 Microsoft has
addressed this somewhat but not entirely, not warning on while(true) for
example.

Prior to this change some C4127 warnings occurred when I built with
Visual Studio using the generated projects in the projects dir.

Closes https://github.com/curl/curl/pull/4658

This is the common pattern used in the code and by a unified approach we
avoid mistakes.

Closes #4534