newvers.sh: add option to eliminate kernel build metadata

Build metadata (username, hostname, etc.) prevents the FreeBSD kernel
from building reproducibly. Add an option to disable inclusion of that
metadata but retain the release information and SVN/git VCS details.
See https://reproducible-builds.org/ for additional background.

Reviewed by: bapt
Obtained from: NetBSD
MFC after: 1 month
Sponsored by: Reproducible Builds World Summit 2, Berlin
Differential Revision: https://reviews.freebsd.org/D4347