Fix the Windows preferences support

Now that I can test Windows binaries again, the bugs were rather easy to
spot. Because of the different flow of the opening, writing and closing of
the registry key my first attempt got things wrong - we simply always
create the key with all access rights; if it exists Windows will just open
it for us. The second bug was a cut'n'paste error.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>