This CL chops existing sticky socket option handlers and puts them
into platform dependent sticky socket option binding table for
supporting multicast features such as source filtering for any-source
multicast, source-specific multicast.

Also adds tiny syscall shims to help to support solaris, to improve
existing platform support.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/140640045