Implement "VARS_ONLY=1" via special command line args

Add -v to print TYPE REVISION BRANCH RELEASE VERSION RELDATE variables
Add -V var to print var's value
Both of these in ${var}="${val}" format suitable for

		eval $(sh newvers.sh -v)

in shell scripts / makefiles.
Add -c to print the copyright / license comment text only.

Document these, and remove soon-to-be obsolete comment.

Minor code motion as well bunded here to put functions after
VARS_ONLY and command line argument parsing.

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