net, init: derive default onion port if a user specified a -port

After port collisions are no longer tolerated but lead to
a startup failure in v28.0, local setups of multiple nodes,
each with a different -port value would not be possible anymore
due to collision of the onion default port - even if the nodes
were using tor or not interested in receiving onion inbound connections.

Fix this by deriving the onion listening port to be -port + 1.
(idea by vasild / laanwj)

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

Github-Pull: bitcoin/bitcoin#31223
Rebased-From: 0e2b12b92a

src/torcontrol.cpp

@@ -711,9 +711,9 @@ void StopTorControl()
     }
 }
 
-CService DefaultOnionServiceTarget()
+CService DefaultOnionServiceTarget(uint16_t port)
 {
     struct in_addr onion_service_target;
     onion_service_target.s_addr = htonl(INADDR_LOOPBACK);
-    return {onion_service_target, BaseParams().OnionServiceTargetPort()};
+    return {onion_service_target, port};
 }