Picking a ruleset on pregame page overrides selected nation

[jwrober]

Describe the bug If you are starting a new game and work from top to bottom on the left, you can override pre-set settings such as nation

To Reproduce Steps to reproduce the behavior:

1. Start a new game
2. Pick a nation other than Random
3. Pick a ruleset other than default (e.g. experimental)
4. Notice the nation is now back to Random.

Expected behavior When a user picks something, its not overridden by another action

Screenshots None

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: master at https://github.com/longturn/freeciv21/commit/51fea1b5f0cedc9361709a574c9d062b6f6f7f7c (but assume its in beta.7)
• Ruleset/Longturn game (if applicable): n/a

Additional context None

[lmoureaux]

Closely related to #1233.

[lmoureaux]

The first action done by the server when switching ruleset is to free all ruleset-related structures it has from the previous ruleset. Since nations are server-defined, there is no way the selected nation can survive this brutal shutdown.