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/misc.c

@@ -739,20 +739,14 @@ void _libssh2_aes_ctr_increment(unsigned char *ctr,
     }
 }
 
-#if !defined(WIN32) && !defined(HAVE_MEMSET_S)
+#ifdef LIBSSH2_MEMZERO
 static void * (* const volatile memset_libssh)(void *, int, size_t) = memset;
-#endif
 
-void _libssh2_explicit_zero(void *buf, size_t size)
+void _libssh2_memzero(void *buf, size_t size)
 {
-#ifdef WIN32
-    SecureZeroMemory(buf, size);
-#elif defined(HAVE_MEMSET_S)
-    (void)memset_s(buf, size, 0, size);
-#else
     memset_libssh(buf, 0, size);
-#endif
 }
+#endif
 
 /* String buffer */