Add a tunable which changes mincore(2) algorithm to only report data
from the local mapping.

Enable the setting by default.
The article behind the change: https://arxiv.org/abs/1901.01161

Reviewed by: markj
Discussed with: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18764