crypto/tls: add Config.Clone

crypto/tls: add Config.Clone

In Go 1.0, the Config struct consisted only of exported fields.

In Go 1.1, it started to grow private, uncopyable fields (sync.Once,
sync.Mutex, etc).

Ever since, people have been writing their own private Config.Clone
methods, or risking it and doing a language-level shallow copy and
copying the unexported sync variables.

Clean this up and export the Config.clone method as Config.Clone.
This matches the convention of Template.Clone from text/template and
html/template at least.

Fixes #15771
Updates #16228 (needs update in x/net/http2 before fixed)
Updates #16492 (not sure whether @agl wants to do more)

Change-Id: I48c2825d4fef55a75d2f99640a7079c56fce39ca
Reviewed-on: https://go-review.googlesource.com/28075
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>