Hijacker again buildable by soviets
Restored production queue current item checks

Fixes regression #5948 where you could freely build any building by
canceling the production before deploying by restoring the currentItem
checks in the PlaceBuilding order to ensure that the production is still
valid.
Merge pull request #5954 from abcdefg30/fixhijacker

Made Hijacker buildable by soviets
Merge pull request #5955 from Squiggles211/production_cancel_fix

Restored PlaceBuilding order's ProductionQueue checks.
Replace the obsolete race check from GiveUnitCrateAction.
Fixed drop-zone units
Merge pull request #5974 from pchote/cratefix

Fix unit collection crates and drop-zone maps.
Adds the RequiresPrerequisites field into ProvidesCustomPrerequisite and all the logic behind it.
Fix aircraft losing selection outside the map borders

Fixes #5651
Merge pull request #5965 from WolfGaming/CustomPrereqChange

Adds the RequiresPrerequisites field into ProvidesCustomPrerequisite
Merge pull request #5967 from obrakmann/fix5651

Fix aircraft losing selection outside the map borders
Fixes gem mine desert palette.
Merge pull request #5987 from reaperrr/gem-mine-palette

Fixes gem mine desert palette
Fix frozen actors being targetable under shroud. Fixes #5979, #5978.
Merge pull request #5989 from pchote/frozen-target-fix

Fix frozen actors being targetable under shroud.
Fix scripted paradrops.
Fix old-lua paradrop too.
Merge pull request #5988 from pchote/shellmap-paradrop

Fix scripted paradrops.
Play the correct race notification when repairing an ally’s structure.
Don’t cancel MoveAdjacentTo / MoveWithinRange when target becomes invalid.

Move to the last known position instead.  Fixes #5968.