Upgrade head ani handling

[MjnMixael]

This adds two new features to head anis to simplify and expand their handling.

First it adds $Always loop head anis: option to messages.tbl. This will cause all heads to loop until the message audio is finished. It doesn't look great with the retail anims because they aren't meant to loop so your mileage may vary. It's meant to be used with the new feature below but was simple enough to make it be its own flag for better customization.

Second it adds $Use newer head ani suffix features: which abandons the old a, b, c method of selecting heads in favor of -reg and -death. In this mode all heads will have -reg appended to the filename unless they are a death scream and not command. (This explicitly allows large ship personas to have death screams, btw) in which case -death will be appended to the filename. In this mode -reg messages will start at a random frame which will provide the same level of variance as the a, b, method provided that the anim itself is sufficiently long enough. -death messages will always start at frame 0 and will not loop.

The idea here is that instead of having 3 versions of a head animation that play until the animation ends, there are only 2 versions; one that plays until the audio is completed (and can loop seamlessly if the audio is longer than the anim) and one that plays on death from start to end.