Fix situations where none of the scripts executed produce output,

so that we don't see any more ``null message body, hope that's
ok'' messages.

We now see something like ``No output from the 3 files processed''.

Lump all output for a given periodic argument together so that
people with /usr/local/etc/periodic/daily (for example) will
get the output of those jobs together with the normal daily run
rather than getting a second email.

Prompted by: ben

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