Change MAYBE-SELECT-BUFFER-LAYOUT so that it does nothing if called

Change MAYBE-SELECT-BUFFER-LAYOUT so that it does nothing if called
while running a layout selector; this will prevent infinite recursions
and that otherwise occur quite easily while instantiating the layout.

Simplify MAYBE-SELECT-BUFFER-LAYOUT so that it doesn't do anything
unless the layout is changing.  Previously indirect tests were used,
now we just compare the current layout with the proposed layout and
implement the changes.