Bugzilla #184: storeDiskdShmGet() assertion

Bugzilla #184: storeDiskdShmGet() assertion

This patch changes the mechanism for keeping track of unused shared
memory chunks.   Instead of using a linked list, now we'll use a
bitmap.  It should eliminate the problems I'm seeing on Alphas
where the linklist->ptr value has bit #33 mysteriously set.  In
the new scheme, the shm chunk pointer is always recalculated based
on the index of the bitmap.