w32: Always use Unicode for console input and output.

* common/init.c (_init_common_subsystems) [W32]: Set the codepage to
UTF-8 for input and putput.  Switch gettext to UTF-8.
* g10/gpg.c (utf8_strings) [W32]: Make sure this is always set.

With this patch the former patch to use ReadConsoleW and WriteConsoleW
in ttyio.c are kind of superfluous because the ANSI version of these
functions are also able to read/write UTF-8 directly given the console
code page has been set correctly. However, this seems to work only
with recent versions of Windows-10.

(cherry picked from commit 8c41b8aac3efb78178fe1eaf52d8d1bbc44941a8)
Removed changes for "gpgconf --show-codepages" of the original patch.

• GnuPG-bug-id: T4365