RPC: Add new getzmqnotifications method.

This adds a new RPC method "getzmqnotifications", which returns
information about all active ZMQ notification endpoints.  This is useful
for software that layers on top of bitcoind, so it can verify that
ZeroMQ is enabled and also figure out where it should listen.

See https://github.com/bitcoin/bitcoin/issues/13526.

src/init.cpp

@@ -62,6 +62,7 @@
 
 #if ENABLE_ZMQ
 #include <zmq/zmqnotificationinterface.h>
+#include <zmq/zmqrpc.h>
 #endif
 
 bool fFeeEstimatesInitialized = false;
@@ -1287,6 +1288,9 @@ bool AppInitMain()
      */
     RegisterAllCoreRPCCommands(tableRPC);
     g_wallet_init_interface.RegisterRPC(tableRPC);
+#if ENABLE_ZMQ
+    RegisterZMQRPCCommands(tableRPC);
+#endif
 
     /* Start the RPC server already.  It will be started in "warmup" mode
      * and not really process calls already (but it will signify connections