dirmngr: Keep track of domains used for WKD queries

* dirmngr/domaininfo.c: New file.
* dirmngr/Makefile.am (dirmngr_SOURCES): Add file.
* dirmngr/server.c (cmd_wkd_get): Check whether the domain is already
known and tell domaininfo about the results.
--

This adds a registry for domain information to eventually avoid
useless queries for domains which do not support WKD.  The missing
part is a background task to check whether a queried domain supports
WKD at all and to expire old entries.

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 65038e6852)

dirmngr/dirmngr.c

@@ -17,6 +17,8 @@
  *
  * You should have received a copy of the GNU General Public License
  * along with this program; if not, see <https://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0+
  */
 
 #include <config.h>
@@ -1871,6 +1873,7 @@ handle_signal (int signo)
 
     case SIGUSR1:
       cert_cache_print_stats ();
+      domaininfo_print_stats ();
       break;
 
     case SIGUSR2: