SPINEL_PROP_NET_REQUIRE_JOIN_EXISTING value

Require Join Existing Format: `b` Default Value: `false` This flag is typically used for nodes that are associating with an existing network for the first time. If this is set to `true` before `PROP_NET_STACK_UP` is set to `true`, the creation of a new partition at association is prevented. If the node cannot associate with an existing partition, `PROP_LAST_STATUS` will emit a status that indicates why the association failed and `PROP_NET_STACK_UP` will automatically revert to `false`. Once associated with an existing partition, this flag automatically reverts to `false`. The behavior of this property being set to `true` when `PROP_NET_STACK_UP` is already set to `true` is undefined.