gpg: New export-filter export-revocs
* g10/options.h (EXPORT_REVOCS): New. * g10/export.c (export_select_filter): New. (struct export_filter_attic_s): Add field. (cleanup_export_globals): Cleanup. (parse_export_options): Add option "export-revocs". (parse_and_set_export_filter): Parse the select type. (do_export_revocs): New. (do_export_stream): Add a way to select things for export.
Backported-from-master: c985b52e71a83d14099f00f80e3588bd3ad28b98