Revise rules for cargo plane loads

Revise rules for cargo plane loads

There are three ways to fly cargo: transport (fly command with a
commodity argument), cargo drops (drop command that isn't a sea mine
drop), and paradrops.

A cargo flight can be either an airlift or an airdrop.  Airlifts carry
more cargo than airdrops.  A cargo drop or paradrop with a non-VTOL
plane is an airdrop.  Anything else is an airlift.

Before, paradrop always behaved like an airdrop, regardless of VTOL,
and drop always like an airlift.  This made little sense.

Effect of the change on the stock game: paradrop with tc carries twice
the punch, and np/tr/jt can drop less than they can fly.  In
particular, tr can't drop guns anymore, and jt can drop only one
instead of three.