Win32: non-ASCII directory names support in ngx_create_dir().

Win32: non-ASCII directory names support in ngx_create_dir().

This makes it possible to create directories under prefix with non-ASCII
characters, as well as makes it possible to create directories with non-ASCII
characters when using the dav module (ticket #1433).

To ensure that the dav module operations are restricted similarly to
other file operations (in particular, short names are not allowed), the
ngx_win32_check_filename() function is used.  It improved to support
checking of just dirname, and now can be used to check paths when creating
files or directories.