preparing for the syscall package freeze.

««« original CL description
syscall: add source-specific multicast socket options for Darwin

Update #8266

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/101460043
»»»

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/115350043