Fix install_name_tool path for keg_only formulae

* When a versioned keg_only formula installs the same set of executables
  or libraries as a unversioned formula that links to $HOMEBREW_PREFIX,
  install_name_tool will prefer to use the linked paths for files in
  keg_only formula.  This breaks software that should link to the
  keg_only formula but links to the unversioned one instead.
* Add an additional keg_only parameter to specify the correct install
  path for keg_only formulae.