Xeno balancing

* Facehuggers become stunned for a short period when thrown. This allows throwing huggers to remain useful for positioning them and setting up traps but not as an offensive ranged weapon.
* Because of the above, helmet facehugger protection is no longer necessary and can be tentatively removed.
* Xenos receive a slight speedup across the board, so that sentinals are now on par with human move speed, drones are a bit slower than humans and hunters are a bit faster. That plus the fixes to the way NO_PAIN affects movement slowdown with damage, should mean xenos are faster overall.
* Weed regeneration occurs at full speed when resting, and is slower when not resting.
* Weed regeneration mends broken bones after all other damage is healed.