Added comment for dummy variable in vehicle.cc.  Enabled _firmwarePlugin call that shows 'Landing aborted.' message.

previously the start log button was always disabled, even for PX4.

Fixes regression from 9d413108

- Shot Interval in Survey
- Much change to time/distance calc

Prevent shutdown ordering crashes

PX4 doesn't retry commands by default

Also changed GPS FactGroup values to use NaN appropriately to show
unavailable values.

altitudeAMSL and altitudeRelative do not need divide 1000，the unit is meter and float format。 can you use _handleGlobalPositionInt message for altitudeAMSL and altitudeRelative？ because _handleGlobalPositionInt is default 5Hz and _handleAltitude is 1Hz.

It's not desired to have automatic log streaming for several reasons:
- There's already SD logging (which is the case for the majority of the
  users)
- The link does not provide the necessary bandwidth (eg. wireless telemetry)

* Moved from UAS
* Remove unused code

Made QGCCorePlugin derive from QGCToolBox and expose everything through the same mechanism as everything else.

The return lead to data being dropped. Some of this data could have been
part of the ulog header, which lead to corrupt files.