This new function allows more flexibility in extending the library to
support more user-configurable options. It is intended to provide a
single API that can support a wide variety of needs and eliminates the
need for new API functions to set future options.

The function is introduced with a single option (LIBUSB_OPTION_LOG_LEVEL)
that replaces the libusb_set_debug() function. Documentation relating to
libusb_set_debug() and the uses of this function in the examples and
tests have been updated accordingly.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

If fdopen(3) fails then ctx->output_file is NULL and should not be used
in cleanup_test_output() called next.

Problem detected by the Coverity tool
CID 1042543 (#1 of 1): Dereference after null check (FORWARD_NULL)9.
var_deref_model: Passing "ctx" to function
"cleanup_test_output(libusbx_testlib_ctx *)", which dereferences null
"ctx->output_file".

* Also fix missing header in release tarball

* fxload sample provenance
* No need for <sys/types.h> in samples as already in libusb.h
* Don't bother about sscanf_s in xusb
* Use HAVE_### and rely on config.h where possible
* Formal inclusion of <winsock.h> in libusb.h for WinCE and WDK
* Cleanup of Windows' config.h
* Avoid ENAMETOOLONG and ENOTEMPTY conflict between errno.h and winsock.h for WinCE
* Additional newlines & braces cleanup

* _dup2 is Windows only. Use dup2 instead
* Breakage was introduced in 76eecc6ce11d1bedee369859899901037dcd4cc2

* cleanup_test_output() produced an assertion error when compiled with MSVC
  due to closing ctx->output_file
* set ctx->output_file to stdout by default