dirmngr: Implement --supervised command (for systemd, etc).

* dirmngr/dirmngr.c (main): Add new --supervised command, which is a
mode designed for running under a process supervision system like
systemd or runit.
* doc/dirmngr.texi: document --supervised option.

"dirmngr --supervised" is a way to invoke dirmngr such that a system
supervisor like systemd can provide socket-activated startup, log
management, and scheduled shutdown.

When running in this mode, dirmngr:

• Does not open its own listening socket; rather, it expects to be given a listening socket on file descriptor 3.
• Does not detach from the invoking process, staying in the foreground instead.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>