gpgtar,w32: Fix gpgtar 8 bit encoding handling on W32
* common/utf8conv.c (wchar_to_utf8): Factor code out to ... (wchar_to_cp): new. (utf8_to_wchar): Factor code out to ... (cp_to_wchar): new. (wchar_to_native): New. (native_to_wchar): New. * tools/gpgtar-create.c (fillup_entry_w32): Use native_to_wchar. (scan_directory): Use wchar_to_native.
Gpgtar needs to handle filenames in the local 8 bit encoding on
Windows as it uses the 8 bit file io functions.
Patch from bug 1624 modified to fit into GnuPG 2.1 by wk.