Rudimentary AER reading code for ddb(4).

This is very primitive code to inspect the PCI error state and AER
error state, dump the log and clear errors, from ddb.
pci_print_faulted_dev() is made external to allow calling it from
other places. It was called from NMI handler but this chunk is not
included.

Also there is a tunable-controlled code to clear AER on device attach,
disabled by default.

All this code was useful to me when I debugged ACPI_DMAR failures (not
faults) long time ago.

Reviewed by: cem, imp (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D7813