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.

Reported by: gjb
Reviewed by: gjb
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8853