monitor: Show combined protection bits in "info mem"

Previously, "info mem" considered and displayed only the last-level
protection bits for a memory range, which doesn't accurrately
represent the protection of that range.  Now it shows the combined
protection.

Signed-off-by: Austin Clements <amdragon@mit.edu>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
