Rework MD5 buildable animation loading.

This loads a few extra animations.

"idle3" → "powerdown"
BANIM_IDLE3 → BANIM_POWERDOWN
  This is used for human buildables when power is lost.
  It is no longer looped.

"destroy2" → "destroy_unpowered"
BANIM_DESTROY2 → BANIM_DESTROY_UNPOWERED
  This is used when an unpowered buildable (human or alien) is destroyed.
Actually use the ‘destroy_unpowered’ animation for MD5 buildables.

The fallback is the normal ‘destroy’ animation: there will therefore be no
change for any given buildable type unless the new animation is defined for
it.