Verify mincore(2) semantics

[bsdjhb]

After a medium-length discussion with Kevin, we have arrived at the following requirements for mincore(2)'s address argument. We should ensure CheriBSD is using these same rules:

https://git.morello-project.org/morello/kernel/linux/-/wikis/Morello-pure-capability-kernel-user-Linux-ABI-specification#mincore