Prepare transfer_city() callers for city destruction.

Reported by Marko Lindqvist <cafzi>

See gna bug #18191

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25666 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Adjustements for path-finding for fueled units:
* better synchronization between the two priority queues
to ensure the nodes to be processed in a good order ;
* safety of a node is taken in account in total_CC
calculation ;
* handle correctly if the unit would implicitly refuel.

See gna patch #4931

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25668 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
- Updated following latest string changes



git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25671 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Make the stub ruleset more minimal by removing all action enablers.

See patch #4959

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25672 a0f10bec-cc02-0410-94fc-a9cfff90b4cd