[Rfc-markdown] 1.6.37: Support "ACM DOIs"

[Carsten Bormann <[e-mail]>]

I know that you shouldn't push new I-D tool versions four days before
the Internet-Draft deadline, but dkg (Daniel Kahn Gillmor) gave me
90 % of the solution for a problem that had been irking me since Nico
Williams pointed it out first, three years ago: What I call "ACM DOIs".
Quite a few other people ran into (and reported) these since.

"ACM DOIs" are identifiers that look like DOIs, but doi.org knows nothing
about them.
But then dl.acm.org does, and that's where we get the goods now.

So you now can cite Usenix conference papers:

  DOI.10.5555_3277203.3277245:
➔
   [DOI.10.5555_3277203.3277245]
              Poddebniak, D., Dresen, C., Müller, J., Ising, F.,
              Schinzel, S., Friedberger, S., Somorovsky, J., and J.
              Schwenk, "Efail: breaking S/MIME and OpenPGP email
              encryption using exfiltration channels", USENIX
              Association, Proceedings of the 27th USENIX Conference on
              Security Symposium pp. 549-566, ISBN 9781931971461, August
              2018.

...and Addison Wesley books:

  DOI.10.5555/206746:
    display: Stevens
➔

   [Stevens]  Stevens, W. and G. Wright, "TCP/IP illustrated (vol. 2):
              the implementation", Addison-Wesley Longman Publishing
              Co., Inc., ISBN 020163354X, February 1995.

A big thank you for all the bug reports, John Levine who gave us a theory of what was going on, and now dkg for most of the solution!

Enjoy!

Grüße, Carsten