This refactoring gets rid of the global variables 'forward',
'right', 'up', and 'muzzle' used throughout the file.

Thanks to global variable abuse, `muzzle` was never calculated when
calling UpdateZapEffect via G_UpdateZaps. Each frame, the origin of the
zap entity was set to `muzzle`, which contained a garbage value from a
random player. For the zap entity, the origin's only purpose is to
determine whether it is in someone's PVS. So the bug caused the zap to
appear and disappear for a given client depending on whether the garbage
value is in its PVS. In a quick test with 5v5, the entity for a single
zap would frequently leave and reenter 5+ times.

The information is now stored in TurretComponent

'lifetime' is a required attribute, defined for all missiles. Represents
the amount of time after which a missile explodes/disappears if it
doesn't hit anything.

Generic code using this will come later (for now it's accessed
through the attributes of a hard-coded missile type each time).

It turns out that GL_QUADS is not a valid type for glDrawElements, so
quads mode didn't draw anything.

I tested this by making a direct call to duDebugDrawBox. Not sure if
there is any way to trigger this with navedit-related commands.

The texture coordinate (uv) versions of vertex() are never called
anywhere, and it wouldn't do anything if they were as there is no
way to set a texture.

A behavior tree can now have a prelude selectClass { ... } which takes
decisions about what to spawn as. An extra behavior tree is parsed
inside the block. When executing this tree the spawnAs statement can be
used to change the class selection.

This could already happen e.g. if action suicide was used in the same
frame.

Regression in c4d3db8 "Don't give
spectators WP_ALEVEL0"

This fixes the alien HUD being inappropriately drawn at various times,
e.g. for spectators after a map_restart.

Also remove the onSpawn event type for BSP entities. This seems to be a
remnant of Q3; you can't spawn from a map entity in Unvanquished.

The case I saw is that a bot carrying the painsaw decided to purchase
armor instead. It couldn't do it because it did not sell weapons first.

It seems that this code would also be wrong in the case that a bot is
carrying an upgrade and decides it no longer wants it (it wouldn't sell
the upgrade)

This cvar can display the currently running action node for a selected
bot's client number.

Also remove two gentity_t fields which were only used for missiles and
get them from the missileAttributes_t instead.

Reverts the PAINSAW_RANGE + 60 change of 3145e8e because it plays
badly with this logic:
	if ( inAttackRange && self->botMind->goal.getTargetType() == entityType_t::ET_BUILDABLE )
	{
		BotStandStill( self );
	}

Implement the intended idea of not *always* firing the painsaw, but
rather only when in the vicinity of an enemy player (or in range
of a buildable).