Handle the save_yourself event from a session manager.
A session manager can tell Emacs that the window system is shutting down
by sending Emacs a save_yourself message. Emacs executes this function when
such an event occurs. This function then executes `emacs-session-save'.
After that, this function informs the session manager that it can continue
or abort shutting down the window system depending on the return value
from `emacs-session-save' If the return value is non-nil the session manager
is told to abort the window system shutdown.
Do not call this function yourself. (fn EVENT)