As its name suggests, allow_shutdown_save aims to authorize session
saving, i.e. not to force it, but on the contrary to prohibit it
exceptionally, for example when using the --fast option of
xfce4-session-logout.

In addition, there is no longer any reason to retain a return value for
session saving in xfsm_logout_dialog(), since there is no longer a
checkbox that can modify the decision to be taken.

Completes: da19807f
Related: #105 (comment 86446)