[Nmh-workers] Removing defensive copy on return

[Joel Reicher]

If people agree, I'd like to go through the nmh code replacing defensive
copying on return with a const return value, and of course go through
the rest of the code fixing any warnings about constness being casted
away.

I think defensive copying is the right thing in many situations, but not
for returned data, since it might be a completely unnecessary overhead,
and it also makes memory management hard to comprehend since the allocation
takes place in a different part of the code than the freeing.

Cheers,

        - Joel