nvdimm(4): Export NVDIMM health flags via sysctl

The ACPI NFIT specification defines a set of "NVDIMM State Flags". These
flags are already reported by `acpidump -t', but this change makes them
available on a per-device basis, in a format that is more easily parsed.

To simplify this, introduce acpi_nfit_get_memory_maps_by_dimm(), which
locates the (ACPI_NFIT_MEMORY_MAP)s associated with a given
(nfit_handle_t).

Reviewed by: mav, cem
Tested by: mav, rpokala (version for stable/12)
MFC after: 3 days
Sponsored by: Panasas