dirmngr: Switch to an onion address if Tor is running.

* dirmngr/dirmngr.h (opt): Turn field 'keyserver' into an strlist.
* dirmngr/dirmngr.c (parse_rereadable_options): Allow multiple
--keyserver options.
* dirmngr/server.c (server_local_s): Add field 'tor_state'.
(release_uri_item_list): New.
(release_ctrl_keyservers): Use it.
(start_command_handler): Release list of keyservers.
(is_tor_running): New.
(cmd_getinfo): Re-implement "tor" subcommand using new fucntion.
(ensure_keyserver): Rewrite.
* g10/dirmngr-conf.skel: Add two keyserver options.
--

This feature is independent of --use-tor and automagically uses Tor if
available.  The dirmngr.conf file needs to specify two keyservers to
make this work.  For new installations this is done using the skeleton
file.  This feature requires the Libassuan 2.4.2 to work.

This patch also fixes a memory leak of opt.keyserver en passant.

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

dirmngr/dirmngr.h

@@ -131,7 +131,7 @@ struct
   unsigned int ocsp_current_period; /* Seconds a response is considered
                                        current after nextUpdate. */
 
-  char *keyserver;    /* Malloced string with the default keyserver.  */
+  strlist_t keyserver;              /* List of default keyservers.  */
 } opt;