build: add support for clang-cl, add CI job

- ci/appveyor: add clang-cl job.
- ci/appvayor: optimize setting an env.
- build: fix clang-cl builds.
- build: fix `-Wcast-function-type` compiler warnings for OpenSSL 3.
- build: use `stdint.h` with MSVC when supported.
- src: use `PRId64` for MSVC where supported.
- src: avoid recursive macro definition for `recv()` and `send()`.
- session: silence `-Wcast-function-type` for `libssh2_session_callback_set2()`.
  Sadly this function is still not fully warning-clean, and it
  seems we'd need separate setter-getters for each callback
  to avoid all warnings.

Closes #1484

example/x11.c

@@ -347,8 +347,15 @@ int main(int argc, char *argv[])
         libssh2_trace(session, LIBSSH2_TRACE_CONN);
 
     /* Set X11 Callback */
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wcast-function-type"
+#endif
     libssh2_session_callback_set2(session, LIBSSH2_CALLBACK_X11,
                                   (libssh2_cb_generic *)x11_callback);
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
 
     /* Authenticate via password */
     rc = libssh2_userauth_password(session, username, password);