libsysdecode: Include required headers in sysdecode.h

[markj]

Summary

It seems a bit silly to force all consumers to include several system
headers any time they include sysdecode.h. There has been a trend in
FreeBSD to make headers more self-contained, so let's apply that here.
No functional change intended.

[emaste]

Sounds good to me

[pauamma_gundo.com]

Manual page changes appear consistent with addition to sysdecode.h. (Naive question: does always including stdio.h cause any noticeable bloat?)

[markj]

In D34899#791172, @pauamma_gundo.com wrote:

(Naive question: does always including stdio.h cause any noticeable bloat?)

I'm not sure what you mean by bloat. The only negative effect of this is that a consumers which already includes stdio.h will force the C preprocessor to read the file again. This has no impact on the compiled code but forces the C preprocessor to read the file again.