- routing header declaration with RFC3542
(note: sizeof(ip6_rthdr0) has changed!)
also, sync up with RFC2460 routing header definition (no "strict" source
routing mode any more)
part of advanced API update (RFC2292 -> 3542).
- keep the case consistent between the actual name and what's referenced.
e.g, if it's `foo', don't use '.Nm Foo' at the start of a sentence.
- remove unnecessary `.Nm foo' after the first occurrence (except for
using `.Nm ""' if there's stuff following, or for the 2nd and so on
occurrences in a SYNOPSIS
- use Sx, Ic, Li, Em, Sq, and Xr as appropriate