portage.update.update_config_files(): Add handling of:
/etc/portage/profile/packages
/etc/portage/profile/package.use.force
/etc/portage/profile/package.use.mask
/etc/portage/profile/package.use.stable.force
/etc/portage/profile/package.use.stable.mask
portage.update.update_config_files(): Add handling of /etc/portage/sets.