sysctl() bugs

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I just had occasion to learn how sysctl hw.iostats works on 4.0.  This
has pointed up a bug in sysctl(), though whether the code or the doc is
not clear to me.  The manpage says the return value on success is the
number of bytes copied out, but I'm seeing zero on success.  (The byte
count is being stored through the data-length pointer correctly.)

Which is right?  (I can generate a PR, but want to know whether it's
sw-bug or doc-bug.)

/~\ The ASCII                           der Mouse
\ / Ribbon Campaign
 X  Against HTML                mouse%rodents-montreal.org@localhost
/ \ Email!           7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B