First attempt to bring branch in sync with HEAD
Add remark about buf's union b_u in front of struct buf and its uses; can
possibly be removed later altogether.

Add remark uncommenting struct cluster_save that is not used in active code
anymore; its an FFS specific part that was uncommented out 2 years ago. It
was used in ffs_reallocbuf() wich has bitrotted a lot and is in the current
state not UFS2 compatible.
Remove direct references to buf->b_un.b_addr instead of using buf->b_data.
This allows UVM- or other structures to be used eventually.
Move the structure `cluster_save' to the dead ufs/ffs code that was using
it solely.

Preserved just in case the code is resurrected one day.
Move declaration of b_data up just under that of the union it refers too as
done in the other unions.
Reorder struct buf a bit more logically
Pullup to -current
Pullup to HEAD