Change gpgme-w32-spawn to unicode

* src/Makefile.am (gpgme_w32spawn_CFLAGS): Add -municode.
* src/gpgme-w32-spawn.c (build_commandline, my_spawn)
(translate_handles): Convert to wchar_t API.
(main): Use wmain instead.

Some time ago we introduced an inconsistency that w32-util called
gpgme-w32-spawn through CreateProcessW but since gpgme-w32-spawn
internally worked with 8 bit the chars were mangled and the
arguments not passed correctly through the CreateProcessA of the
child process. Since the GnuPG processes use GetCommandLineW
this is the proper way to pass on Unicode command line arguments.

Please note that we did not pass UTF-8 before this patch but
rather some broken native encoding where Windows replaces
unicode characters with question marks etc.

• GnuPG-Bug-Id: T6728