Win32: removed NGX_DIR_MASK concept. Previous interface of ngx_open_dir() assumed that passed directory name has a room for NGX_DIR_MASK at the end (NGX_DIR_MASK_LEN bytes). While all direct users of ngx_dir_open() followed this interface, this also implied similar requirements for indirect uses - in particular, via ngx_walk_tree(). Currently none of ngx_walk_tree() uses provides appropriate space, and fixing this does not look like a right way to go. Instead, ngx_dir_open() interface was changed to not require any additional space and use appropriate allocations instead.