Fix path lookup stuff once more

Fix path lookup stuff once more

Sigh.

realpath is "portable", except not implemented on older versions of
coreutils, including the one in Ubuntu Trusty :-(

So, default to using realpath (which has the advantage of not requiring
options anywhere), and fall back to readlink -f if that doesn't exist in
our path.

If that also fails, then the test suite will fail, but have a nice
"readlink: -f: invalid option" or some such all over it, pointing to
what the likely culprit is.