[Nmh-workers] update to default mhshow rule for PostScript attachments

[David Levine]

> Again, that's an issue with '|'

That reminds me, I removed this from mhn.defaults recently:

  mhshow-show-application/PostScript: %plpr -Pps

(or lp -dps if lpr wasn't found).  While PostScript attachments
are probably close to extinct, I didn't see a need for any
potential security risk/embarrassment.

If that's a concern for anyone, you might want to edit your
etc/mhn.defaults.

It's been replaced with a PS viewer, if okular, evince, or
gv is found.

David