Respect slow_invasions in pathfinding.

Patch by Emmet Hikory (persia@gna)

See gna patch #3897

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24220 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Consolidate UTYF_IGTER logic in pathfinding.

Patch by Emmet Hikory (persia@gna) with minor changes by me

See gna patch #3900

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24222 a0f10bec-cc02-0410-94fc-a9cfff90b4cd