Check for readlink / greadlink before trying to use them

Perform a preflight check to ensure that we have readlink or greadlink, exiting with failure if we don't.

readlink comes from GNU coreutils.  On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.

Resolves issue #389