vinum_scandisk: Make sure the config is locked before we mess around

vinum_scandisk: Make sure the config is locked before we mess around
with it.

Finally implement read policies.  The previous "implementation" didn't
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

print_config: Print "preferred" where appropriate.
	      No longer print "prefer" on volume config entries.