Switch reproducible builds to unmodified src tree mode

newvers.sh supports two modes for reproducible builds:

-r Reproducible build. Do not embed directory names, user

names, time stamps or other dynamic information into
the output file.  This is intended to allow two builds
done at different times and even by different people on
different hosts to produce identical output.

-R Reproducible build if the tree represents an unmodified

checkout from a version control system.  Metadata is
included if the tree is modified.

Switch to the second mode when reproducible builds are enabled.
The value of a reproducible build is much less when building from an
uncontrolled, modified src tree, and -R likely provides the best
compromise in allowing the REPRODUCIBLE_BUILD knob to be enabled by
default for the release.

Approved by: re (kib)
Sponsored by: The FreeBSD Foundation