land_safe: rework levels

• Add support for unlimited fuel (-1).
• Add some levels without fuel limits and more easy levels (low gravity) at the beginning.
• Align thrust-levels with accelerometer baseline. It's easier to always have a thrust level that matches the baseline, so the player can be sure that velocity does not change anymore.
• Also remove the "Planet" prefix in the on-screen display because most of our clestial objects are not planets but moons.