ofproto: Implement "hidden" and "readonly" OpenFlow tables.
A "hidden" table is one that OpenFlow operations affect only if the
table_id is explicitly specified, that is, operations that affect
all tables ignore it.
A "read-only" table is one that OpenFlow operations are not allowed
to modify.
I intend to use these flags in an upcoming commit for implementing
tables internal to the Open vSwitch implementation.