Client-side message checks.
Kill off the HAS_POWER flag.

Pawn::powers.empty() should express the same thing, unless there's
something bad going on.
Solving the mystery of the ZOMBIE PAWNS FROM UNINITILISED MEMORY!!

Pawn::destroyed_by was never being initialised, so recent changes that
I thought would make things more stable ended up causing problems, like
pawns not rendering their power lists.
Pawn doesn't need to be copyable either.