For now, distinguish alignment, translation and permission faults.
The first kind of faults cause the kernel to send SIGBUS to the
process causing the fault, the latter two are forwarded to `vm' as
pagefaults. Previously, any data abort was forwarded to `vm' as
a pagefault, resulting in hard to debug issue #104.
Any unhandled fault status results in a disaster. This seems
better than naively hoping `vm' can do something about it.
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