allow the AI to base-walk without restrictions

fixes #2899
Merge pull request #3219 from pchote/remove-server-pinger

Remove server pinging
Merge pull request #3220 from pchote/remove-sequence-warnings

Remove sequence warnings for the stable release.
Merge pull request #3143 from Mailaender/ai-basewalk

Allow the AI to base-walk without restrictions
Make world-coordinates [Sync]able
Convert Aircraft to use world coordinates internally.
Convert helicopter repulsion to world coordinates.