Improve the 'network_id' configuration option:

The 'network_id' option allows an administrator to specify to which
network they intend a server to connect. Servers can leverage this
information to optimize routing and prune automatically discovered
cross-network connections.

This commit will, if merged:

- add support for the devnet keyword, which corresponds to network ID #2;
- report the network ID, if one is configured, in server_info

src/ripple/overlay/impl/OverlayImpl.h

@@ -359,10 +359,15 @@ public:
         return peerDisconnectsCharges_;
     }
 
+    boost::optional<std::uint32_t>
+    networkID() const override
+    {
+        return networkID_;
+    }
+
     Json::Value
     crawlShards(bool pubKey, std::uint32_t hops) override;
 
-
     /** Called when the last link from a peer chain is received.
 
         @param id peer id that received the shard info.