»give health <amount>« sets health to the given value.

This is my first non-C89-compatible commit to the gamelogic! Yay!
Rework alien health regeneration.

Heal rate changes:
- In the open:       1x (unchanged)
- Inside base:       2x (from 3x)
- Close to teammate: 2x (new)
- On actual creep:   4x (new)
- Close to booster:  8x (from 9x)