how to make dirmngr a singleton

[Hans-Christoph Steiner]

In the Android app, gpg-agent and dirmngr are both started in their own
threads.  They often stay alive when the app itself quits.  gpg-agent seems to
automatically find whether its running and refuse to start a second one,
that's good.  But dirmngr does not seem to do that, so after starting and
stopping the app, there will be multiple dirmngrs running.

I can write code to kill old ones or detect if one is running before starting
another, but I wanted to check first to make sure that there isn't already a
way to do that.

.hc