This change consolidates functions and methods related to TCPAddr,
TCPConn and TCPListener for maintenance purpose, especially for
documentation. Also refactors Dial error code paths.

The followup changes will update comments and examples.

Updates #10624.

Change-Id: I3333ee218ebcd08928f9e2826cd1984d15ea153e
Reviewed-on: https://go-review.googlesource.com/20009Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>