sys-apps/texinfo: Re-add keywords to -r3 with standalone useflag

sys-apps/texinfo: Re-add keywords to -r3 with standalone useflag

This should now be a complete workaround for bug 638914, at least
as far as texinfo is concerned: with default USE=+standalone,
* texinfo does not install anything linking to libperl anymore
* texinfo installs only pure-perl modules, and only in its own
  library path
* the modules do not depend on anything from the regular perl
  installation except the perl interpreter itself

Bug: https://bugs.gentoo.org/638914
Package-Manager: Portage-2.3.67, Repoman-2.3.14
Signed-off-by: Andreas K. Hüttel <dilfridge@gentoo.org>