gpg: Add import filter "drop-sig".
* g10/import.c (import_drop_sig): New variable. (cleanup_import_globals): Release that. (parse_and_set_import_filter): Add filter "drop-sig". (filter_getval): Implement properties for drop-sig. (apply_drop_sig_filter): New. (import_one): Apply that filter.
- Signed-off-by: Werner Koch <wk@gnupg.org>