* src/ksba.h.in (KSBA_CLASS_ENCAPSULATE): New pseudo class.
* src/der-builder.c (struct item_s): Add field 'encapsulate'. Change
'class' to a 2 bit field.  Decrease size of 'hdrlen' to 10 bits which
is more than enough.
(_ksba_der_builder_reset): Clear 'encapsulate'.
(_ksba_der_add_ptr): Mask CLASS to avoid possible compiler warnings.
(add_val_core): Ditto.
(_ksba_der_add_tag): Ditto.  Set ENCAPSULATE.
(compute_lengths): Account for extra octet.
(_ksba_der_builder_get): Implement encapsulated data.
* tests/t-der-builder.c (test_der_builder): Add test cases for
encapsulated data.