Fixes  #5532 and increases the time required for organs to become infected from wounds

Also removes the now unnecessary get_cure_threshold() proc, caps the
rate at which an organ can receive germs from wounds, and makes germs
spread from external to internal organs happen one organ at a time
instead of all at once.
Makes minor wounds harder to infect
Tweaks internal organ spread rate
Increases cough rate for lung infections
Fixes missing length check before pick()
Fixes advanced trauma kits claiming to disinfect but not doing so
Sterilizine reduces germ_level on contact
Fixed wrong operator
Merge pull request #5533 from mwerezak/wound-infections

Fix for #5532 and the last of the infection changes, I hope