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

[fanquake]

Moved from bitcoin/bitcoin#18184.

On master (68e841e) 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?

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