Move the count of items in the slabheader from raw data to struct

Move the count of items in the slabheader from raw data to struct

The count of items was stored in the raw data as first two bytes.
Instead of reading this from the raw header, move the number of the
items into the structure itself.

This needs the flexible member raw[] to be aligned on the size of the
pointer to prevent unaligned access to the start of the header from
rdataset_getheader() function that casts the raw[] to dns_slabheader_t.