aen@ asked to ship this one as well; no problem given mixins,
still being able to *switch* the init instead of regrafting
would be very beneficial.

NB:
- wdm can't do autologin;
- wdm can poweroff/halt;
- wdm+autologin won't work under systemd (via prefdm.service);
- nodm will work under systemd;
- nodm will ruin consolehelper -> livecd-install by root's PATH;
- gdm2.20 is lightweight, feature complete and sovereign enough.