gpg: Add property "fpr" for use by --export-filter.
* g10/export.c (push_export_filters): New. (pop_export_filters): New. (export_pubkey_buffer): Add args prefix and prefixlen. Adjust callers. * g10/import.c (impex_filter_getval): Add property "fpr". * g10/main.h (struct impex_filter_parm_s): Add field hexfpr.
The push and pop feature will help us to use the export filter
internally in gpg. Same for the export_pubkey_buffer change.
- GnuPG-bug-id: T4856
- Signed-off-by: Werner Koch <wk@gnupg.org>