exec examples

[Brian Candler]

Minor issue:

[raddb/modules/exec]
exec {
        wait = no
        input_pairs = request
        shell_escape = yes
        output = none
}

However it looks like "output = none" is spurious since rlm_exec doesn't
accept an "output" parameter. I think it should be "output_pairs = none" ?

(Aside: if I set garbage like "zoutput = none" it's happy with that too)

Regards,

Brian.