Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

libsysdecode: Include required headers in sysdecode.h

Make sysdecode.h self-contained rather than forcing all consumers to
include dependencies. No functional change intended.

Reviewed by: pauamma_gundo.com, jhb, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34899

Add support for decoding the type of a cmsg.