B"H web ui: fix bug in plane updating

The "valid*" plane values should be preserved if had been set.

Setting this as a boolean even when set to a non-boolean value by user
removes values with special meaning, for example
validaltitude == -1 (on-ground).
Merge branch 'web_update'
B"H web ui: fix altitude validity detection
Merge branch 'web_update'
B"H web ui: recode handling of missing data
B"H web ui: make plane update code easier to read
Merge branch 'master' into since
Merge branch 'web_update'
B"H Merge branch 'master' into since

Conflicts:
	public_html/planeObject.js