[tex-live] Add rmpost in shell_escape_commands ??

[Karl Berry]

    Hmm... prefix "r-" instead of "r"? 

Hmm. That looks like a pretty good suggestion to me. r-mpost, r-pmpost,
r-upmpost, seem much less susceptible to misunderstanding.

    It will require rebuilding the
    binaries, so if we can wait for 2019 ...

Clearly no one has been urgently needing this, since it's been a year
and a half since we talked about it :). Can you take care of [u]pmpost?

And, Luigi, for the upcoming release, could you change mpost sources to
recognize $0 = r-mpost as meaning -restricted, like rmpost does now?

Thanks,
Karl