gpg: Avoid multiple open calls to the keybox file.

* g10/keydb.h (KEYDB_HANDLE): Move typedef to ...
* g10/gpg.h: here.
(struct server_control_s): Add field 'cached_getkey_kdb'.
* g10/gpg.c (gpg_deinit_default_ctrl): Release that keydb handle.
* g10/getkey.c (getkey_end): Cache keydb handle.
(get_pubkey): Use cached keydb handle.
* kbx/keybox-search.c (keybox_search_reset): Use lseek instead of
closing the file.
--

Before this patch a "gpg --check-sigs" opened and closed the keybox
file for almost every signature check.  By caching the keydb handle
and using lseek(2) this can be limited to just 2 times.  This might
speed up things on Windows.

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

g10/gpg.c

@@ -2195,6 +2195,8 @@ gpg_deinit_default_ctrl (ctrl_t ctrl)
   tofu_closedbs (ctrl);
 #endif
   gpg_dirmngr_deinit_session_data (ctrl);
+
+  keydb_release (ctrl->cached_getkey_kdb);
 }