Add test helper to the check task

The `helper.sh` was not being distributed, which caused `make distcheck`
to fail. Add it as data so that it's not processed but still
distributed.
Expand on instructions for contributors to get started

These changes:
* add the steps needed to get set up
* update the docs regarding mandatory tests
* clarify release instructions
* fix documentation formatting
Add rcup example of adding new rc files

It wasn't obvious that you could also use rcup with undotted files to add them
as new rc files.

There was also a typo.
Add a release script

The Makefiles were mostly filled with a complex shell script written in
m4sh. Moving that out into a separate script helps debugging and
compatibility, and in general makes life better.

This also improved the DEVELOPERS.md documentation to be more clear
about the steps of a release, including third-party packages.