Tweak the way night-vision googles work

- SEE_TURFS flag removed. Night-vision googles no longer act as free mesons, and instead behave like in real life where they DON'T allow you to see through walls
- Darkness_View extended to 5. This is a pretty solid increase and allows the player to see a fair bit more in full darkness

This mostly nerfs night-vision, but it's still interesting for people who want to be top-sneaky in darkness