ArduPlane:make heading lock upon gnd speed consistent #1466

Job	Run time
build (base)	6m 24s
build (clang)	8m 39s
autotest (sitltest-rover)	18m 14s
autotest (sitltest-sailboat)	3m 5s
autotest (sitltest-balancebot)	7m 51s
	44m 13s

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