Fix cargo giveaway in load, lload, unload, lunload and tend

Fix cargo giveaway in load, lload, unload, lunload and tend

Use unit_give_away() in gift().  This fixes a number of bugs:

* Nukes on planes weren't given away along with the plane.

* Likewise for land units on land units (can't happen in the stock
  game).

* Mission was not cleared by unload land/plane, lunload land/plane,
  and lload plane, except for planes on land units.

* Wing and army were never cleared.

It also happens to suppress information on planes given away along
with their land unit carriers.  Shrug.