Add .up pointer to slabheader

Add .up pointer to slabheader

The dns_slabheader object uses the 'next' pointer for two purposes.
In the first header for any given type, 'next' points to the first
header for the next type. But 'down' points to the next header of
the same type, and in that record, 'next' points back up.

This design made the code confusing to read.  We now use a union
so that the 'next' pointer can also be called 'up'.