Mobs on Fire - Atmos interaction and Bodytemp Damage tweaks

Changes to the Mobs on Fire system and Atmos fire:

- You now catch on fire immediately if you enter a hotspot or a hotspot is generated on top of you. This gives you +(0.5) fire stacks (The cap is 20). Remember, resisting will remove 5 stacks, so you could get through about 10 tiles unprotected without suffering too much damage.

Thoughts behind this: fire_act() used to only be called on every tick, meaning you could run through fire constantly without repercussions unless a tick managed to catch you while you were still in fire. Now, fire is an actual obstacle, and crossing it should be met with consideration to where you are and where you want to be.

- Changes to the burn damage levels: Bodytemp of 400-460 has been reduced to 3 from 4, making hot air less lethal to the player. However, being on fire is significantly more lethal.

Thoughts behind this: On current live, it's about 60 seconds of being on fire to go into crit, which wasn't supposed to happen, I accidently made it very weak when I changed something else. After this change, it's about 37 seconds to crit from full health. Taking away Burn Damage level 3 from body temperature and requiring you to be on fire to hit it is due to the fact that you will effectively never hit 1000 bodytemp without dying in the process. Humans can gain 30 temp per tick, max, according to the default compile options, so you'd die on the way there, making the temperature damage level completely worthless and unaccessed by anyone, so I took it for Fire.

- You now illuminate your surroundings when you are on fire

Requested feature.