units: use numerig signal for killing systemd, to avoid incompatibilities with procps/util-linux kill

units: rework systemd-exit.service to terminate systemd via signal rather than bus This should make session termination more reliable, as D-Bus doesn't have to be around anymore for this to succeed.

units: rename halt/hibernate/kexec/poweroff/reboot/suspend to systed-xxx These service units also execute our own code, hence rename the accordingly and prefix them with systemd-