common: First take on handling Unicode command line args.

* common/w32-misc.c: New.
* common/t-w32-cmdline.c: New.
* common/init.c: Include w32help.h.
(prepare_w32_commandline): New.
(_init_common_subsystems) [W32]: Call prepare_w32_commandline.
* common/Makefile.am (common_sources) [W32]: Add w32-misc.c
(module_tests): Add t-w32-cmdline
(t_w32_cmdline_LDADD): New.

The rules for the command line parser are not cleary specified - if at
all. See the comment in t-w32-cmdline.c.

We can't use the mingw version because that would require to change
all argv handling to be wchar_t and that only for Windows. That would
be too ugly. Parsing the command line into argv by us is much easier
and we can do that only if needed - i.e. if globing is required (we
are prepared for this) or a non-ASCII character has been encountered.
This way we keep things stable and only fix the currently not working
Unicode problem.

• GnuPG-bug-id: T4398