Allow to disable default microcode updates search path with the new
'-n' option.

Look for updates in the default locations only after user-supplied
locations are tried.

If newer microcode files are put into non-standard path, both measures
allow to avoid situation where older update loaded from the default
path first, and then the second update is applied from non-standard
path. Applying intermediate updates might be undesirable.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week