Add set_var() to handle default var assignments and export

This will be needed later by global option handling, and is called from
the existing vars_setup() to clean up hard-coded defaults.
Replace vars source warning with a notice

This change moves to prefer builtin defaults over config file settings,
and makes it apparent where config settings are coming from.

Additionally, an unused die() msg was removed that was never reached
since sourcing doesn't return an exit code.
Rename vars -> vars.example
Add vars.example usage notes
Update README.quickstart with new vars.example info
Update testing-setup.sh dev script for vars.example

Support renamed file