BulletSim: add BSLinkInfo structure to remember link specific information
for each link in a linkset.
Extend BSLinksetConstraint to create and use BSLinkInfo with the default
static constraint.
BulletSim: add implementation of 'physSetLinksetType' and 'physGetLinksetType'
and processing routines in BulletSim.
Add linkset rebuild/conversion routine in BSLinkset.
BulletSim: When converting linkset types, don't try to change the list
of linkset children while iterating through the list.