gpgconf,w32: New debug command --show-codepages.

* tools/gpgconf.c (aShowCodepages): New.
(opts): New command --show-codepages.
(main) [W32]: Implement

This is a Windows helper command to quickly check which code pages are
currently used. Useful for debugging. Has no effect on Unix.

• Signed-off-by: Werner Koch <wk@gnupg.org>