document authselect exit code

[pbrezina]

John Florian, from fedora-devel mailing list:

One last thought, how friendly is this going to be with tools like
puppet and ansible? For example, would something like this be doable?

exec { 'authselect select sssd':
unless => "authselect current | grep -q '^sssd$' && authselect check
| grep -q unmodified"
}

The idea being to only run to make a change if needed to keep change
reports tidy. I can't quite tell at this point because:

$ sudo authselect current
No existing configuration detected.

In this sense, it would be helpful if authselect(8) had some details
about exit codes. Also, the "check" command could be more explicit
about what happens with exit codes/output messages when:

• the config was created by authselect and remains unmodified
• the config was created by authselect but has since been modified
• the config hasn't apparently ever been touched by authselect

Maybe another command like "test" command could be ideal for the job if
it did much the same but gave diff output and suitable exit code
indicating spot-on vs. needs-change.

[pbrezina]

Done. 44fe562