OSPFv3: fix more minor issues

* Add 3 more LS types and the M6-bit.
* Print multicast LS type and prefix option as deprecated.
* Remove needless forward declarations.
* Make ospf6_decode_v3() consider vflag for every packet type and raise
  the margin from "> 0" to "> 1" because it is already "> 0" that far.
* Fix a message to refer to packet type instead of LS type.
* Remove irrelevant comment.
* Don't print unknown protocol version 2nd time.