This change consolidates functions and methods related to UnixAddr,
UnixConn and UnixListener for maintenance purpose, especially for
documentation.

The followup changes will update comments and examples.

Updates #10624.

Change-Id: I372d152099ac10956284e6b3863d7e4d9fe5c8e9
Reviewed-on: https://go-review.googlesource.com/20125
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>