This reverts commit f1739c03e00a51e87bd49d04827e5b765d1c70de.

Add a handler for the PrepareForSleep event on the system bus under
systemd and lock the screen in such an event. This addresses the issue
where "systemctl suspend" suspends the system but the system isn't
locked on resume, with the obvious negative security connotations.

Right now the systemd event handling is not actually working at all.
This happens because _listener_message_path_is_our_session() has code to
check if the event came to our session (e.g. "10") and, under systemd,
does this by trying to compare it with the object path, by stripping the
/org/freedesktop/login1/session/ prefix.

However, under -at least recent versions- of systemd, the session ID
part of the path is escaped separately, and following D-Bus rules
initial numbers are escaped. In the example above, the obj_path is
/org/freedesktop/login1/session/_310, so the comparison actually fails.

Port a fix that has been present on various distributions (e.g. Arch)
and pieces of software (e.g. cinnamon-screensaver) since at least March
2013.  The original author is Peter de Ridder <peter@xfce.org>.

The bug and the fix can both be easily verified by running "loginctl
lock-sessions".