Prevent objects from colliding with own child attachments (#7610)

Also, use a better distance calculation for 'collide with objects'.
Fixes the issue of a vehicle occasionally colliding with its own driver,
causing one of the velocity components to be set to zero.

src/serverobject.h

@@ -173,6 +173,7 @@ public:
 	{}
 	virtual const std::unordered_set<int> &getAttachmentChildIds()
 	{ static const std::unordered_set<int> rv; return rv; }
+	virtual ServerActiveObject *getParent() const { return nullptr; }
 	virtual ObjectProperties* accessObjectProperties()
 	{ return NULL; }
 	virtual void notifyObjectPropertiesModified()