BulletSim: More work on center-of-mass. Remove linksetinfo and rely on simulator to update info.
BulletSim: fix density since the simulator/viewer track density in a
funny unit that is 100 times real density (default 1000).
Fix avatar drifting slowly when stationary flying.
Fix for physical prims getting corrected for being under terrain when it was
just its geometric center that was below terrain.
Add PreUpdatePropertyAction allowing plugable modifiction of phys
parameters returned from Bullet.
Fix an exception setting GravityMultiplier on initialization.
Update DLLs and SOs for good measure (no functional change).