vm_phys: Add a sysctl to dump registered fictitious memory ranges

[markj]

Summary

I've wanted this a couple of times in the past. Save the memattr in
the fictitious memory segment structure so that we can report it from
the sysctl handler, and add conversion routines for each platform.

[kib]

sys/vm/vm_phys.c

372

The why not return "???" from vm_memattr_name()? I would print the numeric value of the unknown memattr.

[markj]

Print a number if memattr decoding fails

[markj]

sys/vm/vm_phys.c

372

The why not return "???" from vm_memattr_name()?

Returning NULL indicates that the memattr was not decoded, it seems better to give that extra information to the caller and let them decide how to handle it.

I would print the numeric value of the unknown memattr.

Ok

[kib]

sys/vm/vm_phys.c

374

I suspect that the width 2 is too narrow, since '0x' from '#' counts there.

[markj]

Fix formatting