http2: send "http/1.1" ALPN in TLS dial in addition to "h2"

RFC 7301 defines the ALPN protocol "http/1.1".

We weren't sending that, so at least one site (dav.box.com) was
rejecting our connection, since they didn't support the only protocol
we advertised ("h2").  Had we advertised nothing, they would've
assumed http/1.1 implicitly.

This CL also hooks up the verbose logging knob to the GODEBUG
environment variable.

Updates golang/go#13598 (fixed when this is copied into std)

Change-Id: I6ea1231d0d0f0bc767caa0458237eefd943d9d3d
Reviewed-on: https://go-review.googlesource.com/17754
Reviewed-by: Ian Lance Taylor <iant@golang.org>
4 files changed