gpg: New option --add-desig-revoker
* g10/gpg.c (oAddDesigRevoker): New. (opts): Add new option. * g10/options.h (opt): Add field desig_revokers. * g10/keygen.c (get_parameter_idx): New. (get_parameter): Make use of get_parameter_idx. (prepare_desig_revoker): New. (get_parameter_revkey): Add arg idx. (proc_parameter_file): Add designated revokers. (do_generate_keypair): Write all designated revokers.