fix stupid 90 degree compass direction hack

The player's ship, heading, etc. is rotated 90 degrees to get a compass
that has 0 degrees being "straight ahead".  This should not be accomplished
that way, instead, the heading should be modified at the last possible
moment before being displayed instead of warping the entire code base
with special case code. (WTF was I thinking?)

Now o->heading, o->tsd.ship.gun_heading, o->tsd.ship.sci_heading
are all in radians, and all using the same system.  Also, the player's
ship's heading and the enemy ship's heading are now using the same
system too.

All in all a ridiculous amount of work to sort this mess out for
exactly zero user-visible changes (except maybe some bugs I
might have failed to notice or introduced).