We are in the situation that gnome-session-binary is the session leader
process for the user. This process is managed via logind and is inside
the session scope of the user. This process has an important role for
the session lifetime management, but we cannot track or manage its state
from the systemd user instance.

This adds a simple protocol to allow us managing the state. The
counterpart is in gnome-session-ctl.c.

It works by creating a named fifo called gnome-session-leader-fifo in
the users runtime directory. The session leader opens it for writing,
the monitoring process opens it for reading.

By closing the FD the monitor process can signal to the leader that the
session has been shut down normally. By writing to the FD the leader can
signal the monitoring process to initiate a clean shutdown of the
session. If either process crashes or is killed, the FD is closed and
the other side will also quit.