build: hand-crafted config rework & header tidy-up

- introduce the concept of a project level setup header
  `src/libssh2_setup.h`, that is used by `src`, `example` and `tests`
  alike. Move there all common platform/compiler configuration from
  `src/libssh2_priv.h`, individual sources and `CMakeFiles.txt` files.
  Also move there our hand-crafted (= not auto-generated by CMake or
  autotools) configuration `win32/libssh2-config.h`.

- `win32` directory is empty now, delete it.

- `Makefile.mk`: adapt to the above. Build-directory is the target
  triplet, or any custom name set via `BLD_DIR`.

- sync header path order between build systems:
  build/src -> source/src -> source/include

- delete redundant references to `windows.h`, `winsock2.h`,
  `ws2tcpip.h`.

- delete unnecessary #includes, update order (`libssh2_setup.h` first,
  `winsock2.h` first), simplify where possible.

  This makes the code warning-free without `WIN32_LEAN_AND_MEAN`.
  At the same time this patch applies this macro globally, to avoid
  header bloat.

- example: add missing *nix header guards.

- example: fix misindented `HAVE_UNISTD_H` `#ifdef`s.

- set `WIN32` with all build-tools.

- set `HAVE_SYS_PARAM_H` in the hand-crafted config for MinGW.
  To match auto-detection.

- move a source-specific macro to `misc.c` from `libssh2_priv.h`.

See the PR's individual commits for step-by-step updates.

Closes #932

cmake/CheckNonblockingSocketSupport.cmake

@@ -74,7 +74,6 @@ int main(void)
 #define WIN32_LEAN_AND_MEAN
 #endif
 
-#include <windows.h>
 #include <winsock2.h>
 
 int main(void)