[win32/registry] Fallback to UTF-8 for unknown codepages

	There are some codepages like cp708 for which no ruby encoding exists:

	    $ ruby -e "Encoding.find('cp708')"
	    Traceback (most recent call last):
	    	1: from -e:1:in `<main>'
	    -e:1:in `find': unknown encoding name - cp708 (ArgumentError)

	win32/registry uses ENCODING to transcode error messages and expand environment
	variables from UTF-16LE, so using UTF-8 seems like the best choice and is better
	than a hard failure.

	This should resolve [Bug #13831]

	Fix memory overread in registry.rb

	The terminator is not actually getting filled in; we're simply passing
	(two) bytes of empty memory as the NUL terminator. This can lead to
	garbage characters getting written to registry values.

	Fix this by explicitly putting a WCHAR_NUL character into the string to
	be sent to the registry API, like we do in the MULTI_SZ case.

	[Bug #20096]
	---
	 ext/win32/lib/win32/registry.rb | 7 ++-----
	 1 file changed, 2 insertions(+), 5 deletions(-)

[Misc #18891]

- ext/win32/lib/win32/sspi.rb: Use fiddle (win32/importer) instead of Win32API