src: enable clear memory on all platforms

- convert `_libssh2_explicit_zero()` to macro. This allows inlining
  where supported (e.g. `SecureZeroMemory()`).

- replace `SecureZeroMemory()` (in `wincng.c`) and
  `LIBSSH2_CLEAR_MEMORY`-guarded `memset()` (in `os400qc3.c`) with
  `_libssh2_explicit_zero()` macro.

- delete `LIBSSH2_CLEAR_MEMORY` guards, which enables secure-zeroing
  universally.

- add `LIBSSH2_NO_CLEAR_MEMORY` option to disable secure-zeroing.

- while here, delete double/triple inclusion of `misc.h`.
  `libssh2_priv.h` included it already.

Closes #810

src/openssl.c

@@ -44,7 +44,6 @@
 #if defined(LIBSSH2_OPENSSL) || defined(LIBSSH2_WOLFSSL)
 
 #include <string.h>
-#include "misc.h"
 
 #ifndef EVP_MAX_BLOCK_LENGTH
 #define EVP_MAX_BLOCK_LENGTH 32