units: use SuccessAction=poweroff-force in systemd-poweroff.service

Explicit systemctl calls remain in systemd-halt.service and the system
systemd-exit.service. To convert systemd-halt, we'd need to add
SuccessAction=halt-force. Halting doesn't make much sense, so let's just
leave that is. systemd-exit.service will be converted in the next commit.

units/meson.build

@@ -97,6 +97,7 @@ units = [
          'sockets.target.wants/'],
         ['systemd-networkd.socket',             'ENABLE_NETWORKD',
          join_paths(pkgsysconfdir, 'system/sockets.target.wants/')],
+        ['systemd-poweroff.service',             ''],
         ['systemd-reboot.service',               ''],
         ['systemd-rfkill.socket',               'ENABLE_RFKILL'],
         ['systemd-tmpfiles-clean.timer',        '',
@@ -179,7 +180,6 @@ in_units = [
         ['systemd-nspawn@.service',              ''],
         ['systemd-portabled.service',            'ENABLE_PORTABLED',
          'dbus-org.freedesktop.portable1.service'],
-        ['systemd-poweroff.service',             ''],
         ['systemd-quotacheck.service',           'ENABLE_QUOTACHECK'],
         ['systemd-random-seed.service',          'ENABLE_RANDOMSEED',
          'sysinit.target.wants/'],

units/systemd-poweroff.service

@@ -13,7 +13,4 @@ Documentation=man:systemd-halt.service(8)
 DefaultDependencies=no
 Requires=shutdown.target umount.target final.target
 After=shutdown.target umount.target final.target
-
-[Service]
-Type=oneshot
-ExecStart=@SYSTEMCTL@ --force poweroff
+SuccessAction=poweroff-force