rename: perform user mode dir loop check when not done in kernel

rename: perform user mode dir loop check when not done in kernel
    Linux performs the dir loop check (rename(a, a/b/c)
    or rename(a/b/c, a), etc.) in kernel. Unfortunately
    other systems do not perform this check (e.g. FreeBSD).
    This results in a deadlock in get_path2, because libfuse
    did not expect to handle such cases.

    We add a check_dir_loop function that performs the dir
    loop check in user mode and enable it on systems that
    need it.