An application using syscall.RawConn in a particular way must take
account of the operating system or platform-dependent behavior.
This change consolidates duplicate code and improves the test coverage
for applications that use socket options.

Change-Id: Ie42340ac5373875cf1fd9123df0e99a1e7ac280f
Reviewed-on: https://go-review.googlesource.com/95335Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>