[patch] pam_exec: use program exit code instead of PAM_SYSTEM_ERR

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Hi,

If the specified program exits with non-zero, current implementation
of pam_exec(8) logs this code and return PAM_SYSTEM_ERR. Therefore,
applications have no idea what went wrong with authentication.

Attached is a patch that changes the behaviour to always return the
program exit code as-is. This lets the program returns meaningful
informations to applications.

I also added a small paragraph explaining this to the man page.

I'm planning to commit this to -CURRENT (maybe in a week or two) and
merge it to 9 and 8 if there're no objections.

Thanks for any comments!

- -- 
Jean-Sébastien Pédron
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