runtime: include constants and defs_*_*.h types in generated Go defs

I had to rename Kevent and Sigaction to avoid the functions of the
same (lowercase) name.

LGTM=iant, r
R=golang-codereviews, r, iant, aram.h
CC=dvyukov, golang-codereviews, khr
https://codereview.appspot.com/140740043