Move to using newvers -c instead of VARS_ONLY=1

Use newvers.sh -c to get the copyright, and newvers.sh -V RELDATE to
get the release date.

Differential Revision: https://reviews.freebsd.org/D19849

include/mk-osreldate.sh

@@ -38,8 +38,8 @@ trap "rm -f $tmpfile" EXIT
 ${ECHO} creating osreldate.h from newvers.sh
 
 set +e
-VARS_ONLY=1
-. "${NEWVERS_SH:=$CURDIR/../sys/conf/newvers.sh}" || exit 1
+COPYRIGHT=$(sh ${NEWVERS_SH:=$CURDIR/../sys/conf/newvers.sh} -c) || exit 1
+eval $(sh ${NEWVERS_SH} -V RELDATE) || exit 1
 set -e
 cat > $tmpfile <<EOF
 $COPYRIGHT