Our `realpath` and `readlink -f` functions (required only because of MacOS,
thanks Steve) were poor substitutes at best.  Mostly they were downright
broken.  This thoroughly overhauls them and adds a test (in comments, since we
don't seem to have shell tests).  For all the interesting cases I could think
of, the fakes act just like the real thing.

Then use those and canonicalize KUBE_ROOT.  In order to make recursive calls of
our shell tool not additively grow `pwd` we have to essentially make the
sourcing of init.sh idempotent.