snprintf: unify fallback logic

Before this patch, the `snprintf()` fallback logic for envs not
supporting this function (i.e. Visual Studio 2013 and older) varied
depending on build tool, and used different techniques in examples,
tests and libssh2 itself.

This patch aims to apply a common logic to libssh2 and examples/tests.

- libssh2: use local `snprintf()` fallback with all build tools.

  We already had a local implementation, but only with CMake. Move that
  to the library as `_libssh2_snprintf()`, and map `snprintf()` to it
  when `HAVE_SNPRINTF` is not set.

  Also change the length type from `int` to `size_t`, and fix
  formatting.

- set or detect `HAVE_SNPRINTF` in non-CMake builds.

  Detect in autotools. Keep existing logic in `win32/libssh2_config.h`.
  Always set for OS/400, NetWare and VMS, keeping existing behaviour.
  (OS/400 builds use a different local implementation)

- examples/tests: drop the CMake-specific fallback logic and map
  `snprintf()` to `_snprintf()` for old MSVC versions, like we did
  before with other build tools. This is unsafe, but should be fine for
  these uses.

- `win32/libssh2_config.h`: make it easier to read.

Closes #812

os400/libssh2_config.h

@@ -212,6 +212,10 @@
 /* Use OS/400 Qc3 */
 #define LIBSSH2_OS400QC3
 
+/* Use our platform-specific local implementation:
+   _libssh2_os400_snprintf */
+#define HAVE_SNPRINTF 1
+
 /* Define to the sub-directory in which libtool stores uninstalled libraries.
 */
 #define LT_OBJDIR ".libs/"