util: Disallow network-qualified command line options

Summary:

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.

This is a backport of Core PR17482

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8739