Refactor PORTAGE_GPG_KEY.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Add preliminary DCO_SIGNED_OFF_BY support for DCO Signed-off-by footer.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
man/make.conf.5: document DCO_SIGNED_OFF_BY

Signed-off-by: Zac Medico <zmedico@gentoo.org>