refs #24842

The avcodec-fast option, if enabled, sets the AV_CODEC_FLAG2_FAST
option. Using AV_CODEC_FLAG2_FAST can cause use of unsafe code in
avcodec which could lead to crashes.

Backport a UNICODE fix that was merged after the GnuTLS code freeze.

Forbid call to GetHandleInformation(). fcntl never goes in that code section in
Winstore builds. It's only used for linux random number generation.

Do not allow calling SecureZeroMemory which is forbidden which is forbidden
and not available as an inline function in some cases.

This is the proper counterpart to windowsapp.

Now that we have a proper Docker image to build it:
registry.videolan.org/vlc-debian-llvm-uwp:20200603145315

A recent mingw64 8 (unreleased) is needed to make use of this. It's available
in our Docker images and in msys2 (although it's using msvcrt so it will
probably fail to link properly)

The forced -lwindowsappcompat is added like the other LDFLAGS in configure.ac.

The other UCRT builds might work in win7.

-luuid is needed as we use IID_IUnknown

Add missing $(AM_LIBADD) messing up the order of libraries

Fixes this warning with clang10:
incompatible pointer types passing 'u_long *' (aka 'unsigned long *') to parameter of type 'uint32_t *' (aka 'unsigned int *')

Similar to what is done in access/smb2.c

As long as at least one client is active, we need to keep trying to
read/write to drain the RX buffers or fill the TX buffers.

We still need to poll (with zero timeout) to detect new connections.

Fixes #24824.

In case of TLS, the TCP socket poll state does not necessarily match
the TLS stream state: data may be in the library-side TLS buffers.
So we need to try to read/write regardless of the socket events.

Refs #24824.

Account for actual data read or written, rather than iteration of the
poll loop.

Note: This won't count TLS handshake progress as activity. But if
the client cannot complete the handshake within a time-out interval,
we might as well drop it.

So the caller knows if the client needs to be polled or not.

If the underlying stream failed, there's no point processing an
outstanding request. It's most likely incomplete, and in any case, we
won't be able to send the response.

That corner case only made sense for connections half-closed on read
end. (Even then, it's a little questionable, because half-closed
connections eventually time out if not fully closed.)