passwd: document exit code when PAM has errored

passwd: document exit code when PAM has errored

closes #1219

When pam returns an error, we were exiting with exit code 10,
which was hardcoded and not documented.  Create a name for it,
and document it in the manpage.

Signed-off-by: Serge Hallyn <serge@hallyn.com>
Reported-by: Marc Haber <githubvisible@zugschlus.de>
Reviewed-by: Alejandro Colomar <alx@kernel.org>