realpath(3) requires the last component of its input argument to be either
absent or accessible by the user requesting the service. The man page
doesn't mention the accessibility (it is more an implementation artifact
actually).

This breaks umount(8) when a remote filesystem does not give access to uid 0,
ie. an error is returned while realpathing the mount point. I am not sure
whether to add special case code to umount.c or to look into the implementation
of realpath. Note the difference with lpath(2) in this respect.

-pk