newvers.sh: add options to eliminate kernel build metadata

MFC r310112, r310114, r310273, r310279

r310112: 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.

r310114: newvers.sh: correct typo in comment

r310273: newvers.sh: add -R option to include metadata only for

unmodified src tree

r310279: newvers.sh: consider as modified SVN mixed revision and other cases

The newvers -R option is intended to include build metadata (e.g. user,
host, time) if the build is from an unmodified VCS tree. For subversion
it considered a trailing 'M' as an indication of a modified tree, and
any other version string as modified.

Also include mixed revision checkouts (e.g. 123:126), switched (123S)
and partial (123P) working copies as modified: the revision number is
insufficient to uniquely determine which source was used for the build.