preparing for the syscall package freeze.
the change for issue 8218 is only applied to go.sys/unix.

««« original CL description
syscall: implement setresuid(2) and setresgid(2) on OpenBSD/FreeBSD/DragonflyBSD

Fixes #8218.

LGTM=iant
R=golang-codereviews, iant, minux
CC=golang-codereviews
https://golang.org/cl/107150043

»»»

LGTM=r
R=r, iant, golang-codereviews
CC=golang-codereviews
https://golang.org/cl/138840044