OSPFv3: add crypto authentication (RFC6506) support

* Update ospf6_print() to distinguish packet length and OSPFv3 data
  length.
* Fix ospf6_print_lshdr(), ospf6_print_lsa() and ospf6_decode_v3() to
  print the variable-sized part of DBDesc, LS-Upd and LS-Ack packets
  with the declared length of the OSPFv3 data in mind.
* Implement as little LLS decoding in ospf6_print_lls() as necessary to
  indicate and skip an LLS data block.
* Implement normal AT decoding in ospf6_decode_at().
* Add ospf6_decode_v3_trailer() to couple the above with ospf6_print().

This commit doesn't include a sample capture as the only existing RFC6506
implementation is not yet ready to serve as a reference.