Fix path munging funcs and usage

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.

hack/lib/util.sh

@@ -18,12 +18,6 @@ kube::util::sortable_date() {
   date "+%Y%m%d-%H%M%S"
 }
 
-# this mimics the behavior of linux realpath which is not shipped by default with
-# mac OS X
-kube::util::realpath() {
-  [[ $1 = /* ]] && echo "$1" | sed 's/\/$//' || echo "$PWD/${1#./}"  | sed 's/\/$//'
-}
-
 kube::util::wait_for_url() {
   local url=$1
   local prefix=${2:-}