work because it referred to plexes which were almost invariably when
referred to. Instead, deprecate the "prefer" keyword for volumes
(though it's still there for the moment) and add a keyword "preferred"
to the plex definition. The relationship is like this:
Old:
vol foo ... prefer foo.p3
New:
plex foo.p3 volume foo preferred
give_plex_to_volume: set preferred plex if specified on plex
definition entry. This involves adding a parameter to the function to
specify the preferred plex.
config_plex: Implement preferred keyword.
780bedeaa5