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>

doc/dirmngr.texi

@@ -85,6 +85,13 @@ Run in background daemon mode and listen for commands on a socket.
 Note that this also changes the default home directory and enables the
 internal certificate validation code.  This mode is deprecated.
 
+@item --supervised
+@opindex supervised
+Run in the foreground, sending logs to stderr, and listening on file
+descriptor 3, which must already be bound to a listening socket.  This
+is useful when running under systemd or other similar process
+supervision schemes.
+
 @item --list-crls
 @opindex list-crls
 List the contents of the CRL cache on @code{stdout}. This is probably