[eclass] add init.eclass, containing constants and variables for eselect init consumers
[app-admin/eselect-init] use init.eclass
[sys-apps/sysvinit] use init.eclass
[sys-apps/sysvinit] re-add all ~arches
[app-admin/eselect-init] add all sysvinit ~arches
[sys-apps/systemd] add an eselect-init compatible ebuild of systemd