Pathfinding refactoring. Main changes are that "can_invade" is now hard-coded,
more calculated values are cached, action handling (for attack, diplomat
actions and trade route establishing) is now open for more control.

See gna patch #4889

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25467 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
pf_parameter now has a unit_type pointer field. It replaces the former
unit_flags, uclass and unknown_MC fields.

See gna patch #4910

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25468 a0f10bec-cc02-0410-94fc-a9cfff90b4cd