http2/h2c: add h2c implementation (unencrypted HTTP/2)

Implements h2c by leveraging the existing http2.Server by implementing the 2
ways to start an h2c connection as described in RFC 7540, which are: (1)
create a connection starting with HTTP/1 and then upgrading to h2c [Section 3.2]
and (2) starting a connection directly speaking h2c (aka starting with prior
knowledge) [Section 3.4].

For both of the above connection methods the implementation strategy is to
hijack a HTTP/1 connection, perform the h2c connection on the hijacked
net.Conn, and create a suitably configured net.Conn to pass into
http2.Server.ServeConn.

For h2c with prior knowledge this is relatively simple. For that we just have
to verify the HTTP/2 client preface has been written to the net.Conn, and
then reforward the client preface to the hijacked connection.

For h2c upgraded from HTTP/1, this is a bit more involved. First we validate
the HTTP/1 Upgrade request, and respond to the client with 101 Switching
Protocols. Then we write a HTTP/2 client preface on behalf of the client,
and a settings frame and a headers frame which correspond to what was in
the upgrade request. Then since http2.Server is going respond with a
settings ACK, we swallow it so that it is not forwarded to the client since
for h2c upgrade from HTTP/1 the 101 Switching Protocols response replaces
the settins ACK.

Fixes golang/go#14141

Change-Id: I435f40216c883809c0dcb75426c9c59e2ea31182
Reviewed-on: https://go-review.googlesource.com/112999
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>