Fixed various xeno issues.

- Fixes ALL simple_animals (not just xenos) dieing whenever they are dead and you hit them again. (Fixed #1444)
- Simple_animal xeno dead sprites are now the same as normal xeno dead sprites.
- *roar and *deathgasp can still be spammed, but a sound is only played every 4 seconds. The actual on-death roar is unaffected.

I'm not sure if there's an issue for *roar and *deathgasp spammability, but if there is please tell me.