Automatically document error codes

Just as SAP creates and populates help from its property wrappers, I believe it would be useful to have a way to document exit codes produced by the command line utility and incorporate them into help.

Even though there are Unix standards like sysexit.h, the developer may wish to:

* Express _which_ exit codes are produced
* Document custom exit codes

The issue names no file, test, or entry point. Start by locating the command-line help generation and exit-code handling, then determine how declared and custom exit codes should be represented in generated help. Done means the utility can express its produced exit codes and include their documentation in help.

Written by the indexing model from the issue text.