Change everything returning `libc::sockaddr_storage` to use an &mut out-...

Replacement for PR #16208 (make sockaddr* struct padding explicit) to make PR #15763 (fix nested returns) merge without breaking TCP code.
I believe the [current conventions](http://aturon.github.io/style/whitespace.html) for a return type on the next line is to line up the arrow with the name of the first argument.