Altitude Hold tweaks

This is a few small changes related to altitude hold. The most important one is 359b322, which resolves an issue when using navigation that the altitude would drop when engaging. In addition, this changes the PID library to not zero the integral when the Ki term is zero. This should be (and is) handled outside the library, but having a Ki of zero does not mean we want to have no integral. The integral term is used as the throttle bias and thus should not be automatically zeroed when there is no Ki.

ed8c069 makes it so that the INS works in indoor mode without setting a home location, and is just for convenience.

I'd like to backport these fixes to master once accepted.