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
Decoding of the third argument depends on the first one. For doing this,
add a corresponding function to libsysdecode.
Thanks to jhb@ for suggesting this.