Rename passive_leader_shares_keep candidate action to leader_shares_keep.
This CA is on by default, dealing with a passive leader is only a special case.
The old syntax still works for backward compatibility (for both CA and macros).

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56029 75b3775d-3801-0410-9e12-e2e43121521d