Rework the gentium and gentium-basic font ports
- Rename gentium to gentium-plus tracking upstream changes
- Use the generic facilities provided in bsd.xorg.mk for handling
font installation, rather than rolling our own. This allows
removing files/fonts.dir from both ports.
- Add LICENSE, and so modify pkg-message not to mention licensing
- Use PLIST_FILES and PORTDOCS rather than a separate pkg-plist file