Port Longturn patches - game mechanics fixes

[daavko]

This is a breakout issue from #503 to track status better.

This is only for game mechanics fixes that don't have a new server option or a ruleset option associated with them.

Short summary of patches Exploit fixes and bugfixes, see commit messages listed below.

Patches to port Commits listed chronologically, beginning from the oldest one.

• longturn/freeciv@1945cd8f3c - Remove exploits of bouncing units due to terrain change and stack conflicts: * Bouncing does not reset UWT; * Bouncing on a native terrain from non-native (TF applied) zeroes mp with slow_invasions set.
• longturn/freeciv@0e98d6216d - Cleanup: Invert city_build_stuff() conditional in update_city_activity()
• longturn/freeciv@0112a87d5b - Fix buildings being sold unexpectedly with gold_upkeep_style Mixed.
• longturn/freeciv@5533fbeaa3 - Fix a crash when a city is destroyed during TC
• longturn/freeciv@61e044c30f - Fix a crash when auto-upgrading unique units