net, gui: -upnp option -- "override" vs "can be changed on-the-fly" #710

Moved from https://github.com/bitcoin/bitcoin/issues/18184.

On master (68e841e0af223a220d1f037e4c5680c1b228aa3e) the -upnp option being set in the command line or in the configuration file overrides the GUI option only on startup.

If a user (un)checks the "Map port using UPnP" checkbox in the "Network" tab of the "Options" window, the client does not take into account the fact that the -upnp option is set in the command line or in the configuration file.

There some ways to resolve an issue:

• document the current behavior
• deactivate the "Map port using UPnP" checkbox if the -upnp option is set in the command line or in the configuration file
• smth else

The brainstorm question is which way is the best?

This issue was initially raised by luke-jr in his review of #18077.