Handle cyclic links smarter in fs.realpath

Rather than aborting in the face of *any* repeated link in a given path,
instead only abort if such a cycle actually makes a given path unresolvable.

Test for this by doing a normal stat.  Still use the seenLinks object to
cache link contents so as to cut own a little bit on readlink calls.

Also add a pathological test that fails without the change to fs.js.