api: update openbsd exceptions
api: update openbsd exceptions

This CL restores dropped constants not supported in OpenBSD 5.5
and tris to keep the promise of API compatibility.

Update issue 7049

LGTM=jsing, bradfitz
R=rsc, jsing, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/94950043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang