gnupg 2.1.17 DNS : macOS lacks nsswitch.conf

[Phil Pennock]

Short version: macOS doesn't use nsswitch.conf, dirmngr breaks.

Longer version:

Running GnuPG 2.1.17 on macOS 10.12.2 for about a week, looks like I
must have failed to restart dirmngr and hadn't actually rebooted in that
time, only slept.  So must have been using 2.1.16's dirmngr -- that's
the only explanation I can find for why things didn't break before.

DNS failures, so I patched the homebrew-versions formula (the build
system) to apply
0001-dirmngr-Strip-root-zone-suffix-from-libdns-cname-res.patch from
Werner's email.  Rebuilt.  Still no joy.

Enabled `log-file` in dirmngr.conf (with `verbose 1024` set), `gpgconf
--kill dirmngr` and retry, to see:

2017-01-18 23:46:22 dirmngr[7945.6] failed to load '/etc/nsswitch.conf': No such file or directory
2017-01-18 23:46:22 dirmngr[7945.6] failed to load '/etc/nsswitch.conf': No such file or directory
2017-01-18 23:46:22 dirmngr[7945.6] resolving 'sks.spodhuis.org' failed: No such file or directory
2017-01-18 23:46:22 dirmngr[7945.6] can't connect to 'sks.spodhuis.org': host not found
2017-01-18 23:46:22 dirmngr[7945.6] error connecting to 'https://sks.spodhuis.org:443': Unknown host
2017-01-18 23:46:22 dirmngr[7945.6] marking host 'sks.spodhuis.org' as dead

So I created a stub `/etc/nsswitch.conf` file, containing just:
  hosts: files dns
then killed dirmngr and tried again, and things worked.

Seems like a simple fix, wondering if I should just get a DCO on file.

-Phil
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