makefs: Ignore the "tags" keyword in mtree manifests

An install using -DNO_ROOT emits mtree entries containing tags used by
pkgbase. makefs(8) can safely ignore them, so do that rather than
emitting a warning for each entry.

Reviewed by: brooks, imp
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29384