Add "persistent=<network id>" line to P2P_PEER ctrl_iface data
if a persistent group credentials are available for this peer.
This makes it easier for external programs to figure out when
a persistent group could be re-invoked with this peer.
For now, this information is only available on the P2P client,
but similar information can be added for GO once a list of P2P
clients is maintained with the persistent group data.