dirmngr: Detect dead keyservers and try another one.
* dirmngr/ks-action.c (ks_action_resolve): Rename var for clarity. (ks_action_search, ks_action_put): Ditto. (ks_action_get): Consult only the first server which retruned some data. * dirmngr/ks-engine-hkp.c (SEND_REQUEST_RETRIES): New. (map_host): Add arg CTRL and call dirmngr_tick. (make_host_part): Add arg CTRL. (mark_host_dead): Allow the use of an URL. (handle_send_request_error): New. (ks_hkp_search, ks_hkp_get, ks_hkp_put): Mark host dead and retry on error.