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/golang.sh

@@ -309,7 +309,7 @@ kube::golang::setup_env() {
   # resultant binaries.  Go will not let us use GOBIN with `go install` and
   # cross-compiling, and `go install -o <file>` only works for a single pkg.
   local subdir
-  subdir=$(pwd | sed "s|$KUBE_ROOT||")
+  subdir=$(kube::realpath . | sed "s|$KUBE_ROOT||")
   cd "${KUBE_GOPATH}/src/${KUBE_GO_PACKAGE}/${subdir}"
 
   # Unset GOBIN in case it already exists in the current session.