Improve support for blacklisting bad memory locations. The user can supply
a text file with a list of physical memory addresses to exclude, and have it
loaded at boot time via the provided example in loader.conf. The tunable
'vm.blacklist' remains, but using an external file means that there's no
practical limit to the size of the list. This change also improves the
scanning algorithm for processing the list, scanning the list only once
instead of scanning it for every page in the system. Both the sysctl and
the file can be unsorted and contain duplicates so long as each entry is
numeric (decimal or hex) and is separated by a space, comma, or newline
character. The sysctl 'vm.page_blacklist' is now provided to report what
memory locations were successfully excluded.

Reviewed by: imp, emax
Obtained from: Netflix, Inc.
MFC after: 3 days