kern: physmem: don't create a new exregion for different flags...

... if the region we're adding is an exact match to one that we already
have. Simply extend the flags of the existing entry as needed so that
we don't end up with duplicate regions.

It could be that we got the exclusion through two different means, e.g.,
FDT memreserve and the EFI memory map, and we may derive different
characteristics from each. Apply the most restrictive set to the
region.

Reported by: Mark Millard <marklmi yahoo com>
Reviewed by: mhorne