AP_NavEKF: Improve robustness to long periods without GPS
AP_NavEKF: Reduce time required to recover from GPS timeouts

The time required for GPS to be lost or rejected before vehicles with airspeed sensors either reset to GPS or invoke the zero side-slip assumption to constrain drift has been reduced from 15 to 10 seconds
A duplicate zeroing of the GPS position offset has been removed
If the vehicle is a non hovering vehicle (eg a plane) then the speed at which the GPS offset is pulled back to zero after a reset is increased from 1 to 3  m/s

This also improves recovery from bad inertial data for planes
DataFlash: don't write out parameters if log open fails
DataFlash: don't try and open logfile on failure more than once

this prevents a corrupted microSD card from causing a continuous
attempt to open a log file while in flight, which can cause large
scheduler delays

Pair-Programmed-With: Grant Morphett <grant@gmorph.com>
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