kbx: Fix keyboxd searching with multiple patterns.

* kbx/keybox-search-desc.h (struct keydb_search_desc): New flag
name_used.
* common/userids.c (classify_user_id): Set flag.
* kbx/kbxserver.c (struct search_backing_store_s): New.
(cmd_search): use a backing store for the const pointers.
(kbxd_start_command_handler): Release the backing store.

Well, the search object partly uses buffers but also const
pointers (for strings and the serial number). This when assigning
such objects to an another one we should really take a deep copy and
not just copy the pointer. The more clean solution would have been to
provide a storage option the search object but that needs checking the
code at too many places so that I decided to use a separate backing
store array here.

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