On multiplexed connections, transfers can be removed from anywhere not
just at the head as for pipelines.

... as the usage needs to be counted.

Commit 38203f15 changed engine detection to be version-based,
with a baseline of openssl 1.0.1. This does in fact break builds
with openssl 1.0.0, which has engine support - the configure script
detects that ENGINE_cleanup() is available - but <openssl/engine.h>
doesn't get included to declare it.

According to upstream documentation, engine support was added to
mainstream openssl builds as of version 0.9.7:
https://github.com/openssl/openssl/blob/master/README.ENGINE

This commit drops the version test down to 1.0.0 as version 1.0.0d
is the oldest version I have to test with.

Closes #2732

Original MinGW's w32api has a sytax error in its definition of
CALG_TLS1PRF [0]. Don't use original MinGW w32api's CALG_TLS1PRF
until this bug [1] is fixed.

[0] https://osdn.net/projects/mingw/scm/git/mingw-org-wsl/blobs/d1d4a17e51a2b78e252ef0147d483267d56c90cc/w32api/include/wincrypt.h
[1] https://osdn.net/projects/mingw/ticket/38391

Fixes https://github.com/curl/curl/pull/2721#issuecomment-403636043
Closes https://github.com/curl/curl/pull/2728

Apparently the C => HTML converter on the web site doesn't quite like it
otherwise.

Reported-by: Jeroen Ooms

Closes #2724

... and not the other way around, which this previously said.

Reported-by: Vasiliy Faronov
Fixes #2723
Closes #2726

Reviewed-by: Jakub Zakrzewski
Closes #2715

Follow-up to 82ce4162.

Ref: https://github.com/curl/curl/commit/8272ec5#commitcomment-29646818

MinGW warns:
/lib/vtls/schannel.c:219:64: warning: signed and unsigned type in
conditional expression [-Wsign-compare]

Fix this by casting the ptrdiff_t to size_t as we know it's positive.

Closes https://github.com/curl/curl/pull/2721

Original MinGW's w32api has CryptHashData's second parameter as BYTE *
instead of const BYTE *.

Closes https://github.com/curl/curl/pull/2721

They are not defined in the original MinGW's <wincrypt.h>.

Closes https://github.com/curl/curl/pull/2721

Otherwise, only part of it gets pulled in through <windows.h> on
original MinGW.

Fixes https://github.com/curl/curl/issues/2361
Closes https://github.com/curl/curl/pull/2721

When size_t is not a typedef for unsigned long (as usually the case on
Windows), GCC emits -Wformat warnings when using lu and lx format
specifiers with size_t. Silence them with explicit casts to
unsigned long.

Closes https://github.com/curl/curl/pull/2721

... not the read buffer size, as that can be set smaller and thus cause
a buffer overflow! CVE-2018-0500

Reported-by: Peter Wu
Bug: https://curl.haxx.se/docs/adv_2018-70a2.html

Closes #2718

...but GCC users lose out on TLS 1.3 support, since we can't weak-link
enumeration constants.

Fixes #2656
Closes #2703

Variable 'output_var' is not used and can be removed.
Function 'collect_true' renamed to 'count_true'.

Closes #2711

... because otherwise not everything get closed down correctly.

Fixes #2708
Closes #2712

Closes #2713

Closes #2706

Closes #2704

telnet.c(1401,28): warning: cast from function call of type 'int' to
non-matching type 'HANDLE' (aka 'void *') [-Wbad-function-cast]

Fixes #2696
Closes #2700

Closes #2698

The code treated the set version as the *exact* version to require in
the TLS handshake, which is not what other TLS backends do and probably
not what most people expect either.

Reported-by: Andreas Olsson
Assisted-by: Gaurav Malhotra
Fixes #2691
Closes #2694

Reported-by: Andreas Olsson
Fixes #2692
Closes #2693

The previous example was a little bit confusing, because SSL* structure
(or other "in use" SSL connection pointer) is not accessible after the
transfer is completed, therefore working with the raw TLS library
specific pointer needs to be done during transfer.

Closes #2690