The previously implementation considered a file to be a mountpoint if
its parent is on a different device. / is its own parent, so by this
definition it is not a mountpoint.

But / is (generally) listed in fstab, and fstab(5) defines the
directories it contains to be mountpoints. This attribute should follow
that definition (and reasonable expectation): the root directory is a
mountpoint.

So, add a special-case for the case where the file's parent has the same
st_dev and st_ino as the file, which is true only at the root.

Test this attribute at / (only on POSIX), /proc (but only on Linux), and
at many files and directories created by the test suite (which cannot be
mountpoints).