w32: Support Unicode also for config files etc.

* common/sysutils.c (gnupg_fopen) [W32]: Use _wfopen if needed.   Use
new function in most places where fopen is used.

The config files in 2.2 are still read using fopen - we need to change
this to allow Unicode directory names. There is also one case where
files are written using the old fopen. The new option parser in 2.3
does not have this problem but at some places fopen is also still used.

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