Add [BulletSim] option AvatarToAvatarCollisionsByDefault to control whether avatars collide.  This is true by default.

This is implemented with a new collision type (PhantomToOthersAvatar) to potentially allow colliding and non-colliding avatars to be present in the same scene.
So there is no provision yet for giving avatars different collision types.
This commit replaces the temporary change in commit f3eaa6d8 where avatars would never collide when using BulletSim
This is equivalent to the av_av_collisions_off option in ODE.