util: Disallow network-qualified command line options

Previously these were allowed but ignored.

doc/release-notes.md

@@ -104,6 +104,13 @@ Wallet
 Low-level changes
 =================
 
+Command line
+------------
+
+Command line options prefixed with main/test/regtest network names like
+`-main.port=8333` `-test.server=1` previously were allowed but ignored. Now
+they trigger "Invalid parameter" errors on startup.
+
 Tests
 -----