dirmngr: Add option --keyserver.

* dirmngr/dirmngr.c (oKeyServer): New.
(opts): Add "keyserver".
(parse_rereadable_options): Parse that options
(main): Add option to the gpgconf list.
* dirmngr/dirmngr.h (opt): Add field "keyserver".
* dirmngr/server.c (ensure_keyserver): New.
(make_keyserver_item): New.  Factored out from
(cmd_keyserver): here.  Call ensure_keyserver.
(cmd_ks_search): Call ensure_keyserver.
(cmd_ks_get): Ditto.
(cmd_ks_fetch): Ditto.
(cmd_ks_put): Ditto.
--

This option specifies the keyserver to be used if the client does not
set another keyserver.  We want to fade out the use of --keyserver in
gpg.conf in favor of specifying it here.

Signed-off-by: Werner Koch <wk@gnupg.org>

dirmngr/dirmngr.h

@@ -130,6 +130,8 @@ struct
                                        considered valid after thisUpdate. */
   unsigned int ocsp_current_period; /* Seconds a response is considered
                                        current after nextUpdate. */
+
+  char *keyserver;    /* Malloced string with the default keyserver.  */
 } opt;