makefs: Make it possible to silence warnings about duplicate paths

When generating a VM image from an installworld mtree manifest, makefs
spits out several thousand warnings about duplicate paths in the
manifest. These are harmless and have been around for a long time (see
the phabricator revision for some more details), so let's at least have
a way to make makefs quieter.

Reviewed by: brooks, imp, emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43513

(cherry picked from commit e0deb85065efbcd2123d65f65eb54bd1dcf0588c)