build fixes and improvements (mostly for Windows)

- in `hostkey.c` check the result of `libssh2_sha256_init()` and
  `libssh2_sha512_init()` calls. This avoid the warning that we're
  ignoring the return values.

- fix code using `int` (or `SOCKET`) for sockets. Use libssh2's
  dedicated `libssh2_socket_t` and `LIBSSH2_INVALID_SOCKET` instead.

- fix compiler warnings due to `STATUS_*` macro redefinitions between
  `ntstatus.h` / `winnt.h`. Solve it by manually defining the single
  `STATUS` value we need from `ntstatus.h` and stop including the whole
  header.
  Fixes #733

- improve Windows UWP/WinRT builds by detecting it with code copied
  from the curl project. Then excluding problematic libssh2 parts
  according to PR by Dmitry Kostjučenko.
  Fixes #734

- always use `SecureZeroMemory()` on Windows.

  We can tweak this if not found or not inlined by a C compiler which
  we otherwise support. Same if it causes issues with UWP apps.

  Ref: https://learn.microsoft.com/en-us/previous-versions/windows/desktop/legacy/aa366877(v=vs.85)
  Ref: https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtlsecurezeromemory

- always enable `LIBSSH2_CLEAR_MEMORY` on Windows. CMake and
  curl-for-win builds already did that. Delete `SecureZeroMemory()`
  detection from autotools' WinCNG backend logic, that this
  setting used to depend on.

  TODO: Enable it for all platforms in a separate PR.
  TODO: For clearing buffers in WinCNG, call `_libssh2_explicit_zero()`,
        insead of a local function or explicit `SecureZeroMemory()`.

- Makefile.inc: move `os400qc3.h` to `HEADERS`. This fixes
  compilation on non-unixy platforms. Recent regression.

- `libssh2.rc`: replace copyright with plain ASCII, as in curl.

  Ref: curl/curl@1ca62bb
  Ref: curl/curl#7765
  Ref: curl/curl#7776

- CMake fixes and improvements:

  - enable warnings with llvm/clang.
  - enable more comprehensive warnings with gcc and llvm/clang.
    Logic copied from curl:
    233810bb5f/CMakeLists.txt (L131-L148)
  - fix `Policy CMP0080` CMake warning by deleting that reference.
  - add `ENABLE_WERROR` (default: `OFF`) option. Ported from curl.
  - add `PICKY_COMPILER` (default: `ON`) option, as known from curl.

    It controls both the newly added picky warnings for llvm/clang and
    gcc, and also the pre-existing ones for MSVC.

- `win32/GNUmakefile` fixes and improvements:

  - delete `_AMD64_` and add missing `-m64` for x64 builds under test.
  - add support for `ARCH=custom`.
    It disables hardcoded Intel 64-bit and Intel 32-bit options,
    allowing ARM64 builds.
  - add support for `LIBSSH2_RCFLAG_EXTRAS`.
    To pass custom options to windres, e.g. in ARM64 builds.
  - add support for `LIBSSH2_RC`. To override `windres`.
  - delete support for Metrowerks C. Last released in 2004.

- `win32/libssh2_config.h`: delete unnecessary socket #includes

  `src/libssh2_priv.h` includes `winsock2.h` and `ws2tcpip.h` further
   down the line, triggered by `HAVE_WINSOCK2_H`.

  `mswsock.h` does not seem to be necessary anymore.

  Double-including these (before `windows.h`) caused compiler failures
  when building against BoringSSL and warnings with LibreSSL. We could
  work this around by passing `-DNOCRYPT`. Deleting the duplicates
  fixes these issues.

  Timeline:
  2013: c910cd382d deleted `mswsock.h` from `src/libssh2_priv.h`
  2008: 8c43bc52b1 added `winsock2.h` and `ws2tcpip.h` to `src/libssh2_priv.h`
  2005: dc4bb1af96 added the now deleted #includes

- delete or replace `LIBSSH2_WIN32` with `WIN32`.

- replace hand-rolled `HAVE_WINDOWS_H` macro with `WIN32`. Also delete
  its detections/definitions.

- delete unused `LIBSSH2_DARWIN` macro.

- delete unused `writev()` Windows implementation

  There is no reference to `writev()` since 2007-02-02, commit
  9d55db6501.

- fix a bunch of MSVC / llvm/clang / gcc compiler warnings:

  - `warning C4100: '...': unreferenced formal parameter`
  - using value of undefined PP macro `LIBSSH2DEBUG`
  - missing void from function definition
  - `if()` block missing in non-debug builds
  - unreferenced variable in non-debug builds
  - `warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]`
    in `_libssh2_debug()`
  - `warning C4295: 'ciphertext' : array is too small to include a terminating null character`
  - `warning C4706: assignment within conditional expression`
  - `warning C4996: 'inet_addr': Use inet_pton() or InetPton() instead or
      define _WINSOCK_DEPRECATED_NO_WARNINGS to disable deprecated API warnings`
    By suppressning it. Would be best to use inet_pton() as suggested.
    On Windows this needs Vista though.
  - `warning C4152: nonstandard extension, function/data pointer conversion in expression`
    (silenced locally)
  - `warning C4068: unknown pragma`

  Ref: https://ci.appveyor.com/project/libssh2org/libssh2/builds/46354480/job/j7d0m34qgq8rag5w

Closes #808

example/sftp_mkdir_nonblock.c

@@ -7,6 +7,12 @@
  * "sftp 192.168.0.1 user password /tmp/sftp_write_nonblock.c"
  */
 
+#ifdef WIN32
+#ifndef _WINSOCK_DEPRECATED_NO_WARNINGS
+#define _WINSOCK_DEPRECATED_NO_WARNINGS
+#endif
+#endif
+
 #include "libssh2_config.h"
 #include <libssh2.h>
 #include <libssh2_sftp.h>
@@ -36,7 +42,8 @@
 int main(int argc, char *argv[])
 {
     unsigned long hostaddr;
-    int sock, i, auth_pw = 1;
+    libssh2_socket_t sock;
+    int i, auth_pw = 1;
     struct sockaddr_in sin;
     const char *fingerprint;
     LIBSSH2_SESSION *session;