This fixes the stack end detection properly, and verifies that the
stack content printing does not overflow into the next page even
partially.

This is required especially for x86 BIOSes that misalign the stack,
together with the page access debugging that unmaps unused kernel
pages to check for valid accesses.

Architectures with special needs (eg HPPA with stacks that grow up)
can override the kernel stack end test with __HAVE_ARCH_KSTACK_END
if they ever enable the anal slab debugging code.