Cygwin: add transform_chars_af_unix helper

This function is going to be used for transposing sun_path of
abstract sockets.  This also adds a transposition of the NUL
character to tfx_chars since NUL-bytes in abstract socket names
are perfectly valid.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/miscfuncs.h

@@ -96,6 +96,8 @@ transform_chars (PUNICODE_STRING upath, USHORT start_idx)
 		   upath->Buffer + upath->Length / sizeof (WCHAR) - 1);
 }
 
+PWCHAR transform_chars_af_unix (PWCHAR, const char *, __socklen_t);
+
 /* Memory checking */
 int __reg2 check_invalid_virtual_addr (const void *s, unsigned sz);