dirmngr: Make --use-tor work - still leaks DNS.

* dirmngr/dirmngr.c (set_tor_mode): New.
(main, reread_configuration): Call it.
* dirmngr/http.c (http_raw_connect, send_request): Check whether TOR
mode is enabled if the FORCE_TOR flag is given.

The patch for http.c is a sanity check because tor mode is anyway
global as long as the Assuan socket wrappers are used.

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