* src/visibility.c (ksba_cms_add_attribute): New.
* src/visibility.h (ksba_cms_add_attribute): New mapping.
* src/libksba.def: Add new function.
* src/libksba.vers: Ditto.
* src/ksba.h.in: Ditto.
* src/asn1-constants.h (TYPE_PRE_SET_OF): New.
* src/asn1-func.c (_ksba_asn_is_primitive): Add TYPE_PRE_SET_OF.
(_ksba_asn_node_dump): Ditto.
* src/cms.h (struct oidparmlist_s): Add fields 'unprotected' and
  'signeridx'.
(struct ksba_cms_s): Add field attribute_list.
* src/cms.c (ksba_cms_release): Free attribute_list.
(ksba_cms_add_smime_capability): Clear new fields.
(ksba_cms_add_attribute): New.
(build_signed_data_attributes): Implement attribute output.
* src/der-encoder.c (_ksba_der_store_set_of): New.
(set_nhdr_and_len): Handle TYPE_PRE_SET_OF.
(copy_nhdr_and_len): Ditto.
(_ksba_der_encode_tree): Encode TYPE_PRE_SET_OF.