sysexits: Tidy deprecated header and manual

<sysexits.h> was deprecated in the base system in a1432b4 for FreeBSD
8.0 and is retained only for backwards compatibility. Make that clear,
since sysexits(3) suggested using it since it was in style(9) prior
to this.

MFC after: 1 week
Co-authored-by: imp
Fixes: a1432b4
Reviewed by: imp, pauamma (previous version)
Pull Request https://github.com/freebsd/freebsd-src/pull/1195

(cherry picked from commit 5ccaab1797e5bdc404f2299ebaec1265547a4744)