Increases the fcache free list header and footer sizes to 32 bits, supporting larger coalesced free regions. This is required for larger cache unit sizes on >4K-sized-page machines. The only downside is that the minimum fragment size goes from 16 bytes to 20 bytes, but only a few 32-bit fragments are that small in any case so it is a negligible cost.
Tested on a 64K-page machine to confirm it fixes the assert.
Fixes #4434 (closed)