crypto/tls: return a typed error on invalid record headers

crypto/tls: return a typed error on invalid record headers

The user can inspect the record data to detect that the other side is
not using the TLS protocol.

This will be used by the net/http client (in a follow-on CL) to detect
when an HTTPS client is speaking to an HTTP server.

Updates #11111.

Change-Id: I872f78717aa8e8e98cebd8075436209a52039a73
Reviewed-on: https://go-review.googlesource.com/16078
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>