rework how pools with large pages (>PAGE_SIZE) are implemented.

this moves the size of the pool page (not arch page) out of the
pool allocator into struct pool. this lets us create only two pools
for the automatically determined large page allocations instead of
256 of them.

while here support using slack space in large pages for the
pool_item_header by requiring km_alloc provide pool page aligned
memory.

lastly, instead of doing incorrect math to figure how how many arch
pages to use for large pool pages, just use powers of two.

ok mikeb@

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