to return the name/type of subnodes. Added a subconfig command that
returns a type-specific configuration that the client will know what
to do with. At the moment, just ixp is known as a subnode. The subnode
support mostly works off the vnode stuff since otherwise it would be
lot of code duplication. Eventually subnode should replace vnode, and
vnode will become a type of subnode (like ixp).

Also changed usage of control_net to control_iface, and made use of
the "role" slot in ifconfig to determine what interfaces to return
info for.