random: Allow for a Unicode random seed file on Windows.

* random/random-csprng.c (utf8_to_wchar) [W32]: New.
(any8bitchar) [W32]: New.
(my_open): New.  Replace all calls to open with this.

Users with account names having an Unicode character in their name may
now create a random_see file without running into an error. Note
that depending on the code page this used to work but for sure no if
more than one byte is required. For testing I used "Ⓐnne".

• GnuPG-bug-id: T5098
• Signed-off-by: Werner Koch <wk@gnupg.org>