Create cache listing extra types that can provide tile nativity to the unit class
at the ruleset loading time. Iterate over that list only when checking if tile
is native for a unit.

See patch #4609


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24738 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Reverted accidentally committed, non-final version, of patch #4635


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24741 a0f10bec-cc02-0410-94fc-a9cfff90b4cd