Handle suspend, shutdown, reboot, etc within elogind

Handle suspend, shutdown, reboot, etc within elogind

Since we are catching the keys, we might as well just do
suspend/reboot/etc handling here.

* configure.ac: Get paths of halt and reboot.
* Makefile.am (systemsleepdir, systemshutdowndir): New variables.  Look
  in them for hooks to run.

* src/login/logind-action.c: Inline the salient bits from systemd's
  sleep/sleep.c here.

* src/login/logind-dbus.c (execute_shutdown_or_sleep): Call our own
  shutdown_or_sleep helper instead of invoking a systemd method.

* src/login/logind-action.h: Declare shutdown_or_sleep.