EKF fallback changes

This pull request addresses a couple issues with EKF-to-DCM fallback:

1. It will fall back to DCM if the EKF thinks the horizontal or vertical position or velocity states are bad. I don't think that's a reason to switch to a less-robust algorithm for attitude estimation.

2. Seems we are always using DCM's attitude in get accel_ef. This affects:
- AC_PosControl vertical acceleration controller
- AP_TECS vertical complementary filter
- AP_InertialNav vertical complementary filter

Could use review by @priseborough and @tridge - I've testflown the changes before but not for quite a while, they've been sitting in a branch getting stale.

Also a question: is starting EKF without GPS possible? Going to be important for "indoor mode" on copters.