dfs_win32: add seekdir support, fix #346

[prife]

test case

int testdir()
{
    DIR * dir;
    struct dirent * ptr;
    int offset = 0, offset_5 = 0, i = 0;
    dir = opendir("/");
    while ((ptr = readdir(dir))!=NULL)
    {
        offset = telldir(dir);
        if(++i == 5) offset_5 = offset;
        rt_kprintf("d_name :%s offset :%d \n", ptr->d_name, offset);
    }
    seekdir(dir, offset_5);
    rt_kprintf("Readdir again!\n");
    while ((ptr = readdir(dir))!=NULL)
    {
        offset = telldir(dir);
        rt_kprintf("d_name :%s offset :%d\n", ptr->d_name, offset);
    }

    closedir(dir);
    return 0;
}