AGM_Medical custom events and preventDeath per unit

I know we are in the process of locking down medical before the rewrite, but I wanted to propose for inclusion a couple of internal features that people may find useful.

- Some custom medical events. For the moment only:
  - AGM_Medical_initialized: when the medical state is reset (spawn or respawn)
  - AGM_Medical_knockedOut: when unit get knocked down
  - AGM_Medical_wokeUp: when unit get woke up
  - AGM_Medical_preventedDeath: when medical prevents the death of a unit

- Prevent death can be set or ovewriten for certain units using an object variable `AGM_PreventDeath`.

This events allow the integration of mission side scripts into medical very easily.