Harrier-GR3: auto-hover: use properties to store target height / vertical speed.
Also, don't override auto-hover properties that were pre-set by command line --prop:<name>=<value>.
These changes for example allow target position/height to be set from command line, becoming active after take off.
Authored by: lbrenta 2019-05-30
Parent: [r3984]
Child: [r3986]
