cli operations turn on seq mode automatically when they can
vips_operation_get_flags() added: lets you attach a set of flags to an
operation
flags for now are "nocache" (replacing the old nocache system) and "seqential"
if vips_object_set_argument_from_string() from string sees "seq" flag on the
object for which it is setting the arg, it enables sequential mode
all operations which can run sequentially have been tagged
the operation printer knows about flags and can display them
John Cupitt