This patch chooses different approach to integrate HTTP2 into HTTP curl
stack. The idea is that we insert HTTP2 layer between HTTP code and
socket(TLS) layer. When HTTP2 is initialized (either in NPN or Upgrade),
we replace the Curl_recv/Curl_send callbacks with HTTP2's, but keep the
original callbacks in http_conn struct. When sending serialized data by
nghttp2, we use original Curl_send callback. Likewise, when reading data
from network, we use original Curl_recv callback. In this way we can
treat both TLS and non-TLS connections.

With this patch, one can transfer contents from https://twitter.com and
from nghttp2 test server in plain HTTP as well.

The code still has rough edges. The notable one is I could not figure
out how to call nghttp2_session_send() when underlying socket is
writable.

Add ALPN support when using GnuTLS >= 3.2.0. This allows
libcurl to negotiate HTTP/2.0 for https connections when
built with GnuTLS.

See:
http://www.gnutls.org/manual/gnutls.html#Application-Layer-Protocol-Negotiation-_0028ALPN_0029
http://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg-04

Add ALPN support when using OpenSSL. This will offer ALPN and NPN to the
server, who can respond with either one or none of the two. OpenSSL >=
1.0.2 is required, which means as of today obtaining a snapshot from
ftp://ftp.openssl.org/snapshot/.

See:
http://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg-04
https://github.com/openssl/openssl/blob/ba168244a14bbd056e502d7daa04cae4aabe9d0d/ssl/ssl_lib.c#L1787

For some reason Windows 7 SP1 chooses TLS 1.0 instead of TLS 1.2
if it is not explicitly enabled within grbitEnabledProtocols.

More information can be found on MSDN:
http://msdn.microsoft.com/library/windows/desktop/aa379810.aspx

... and then go through the "normal" HTTP engine.

Check the NPN result before preparing an HTTP request and switch into
HTTP/2.0 mode if necessary. This is a work in progress, the actual code
to prepare and send the request using nghttp2 is still missing from
Curl_http2_send_request().

To better reflect its purpose

NPN is what is available in the wild today to negotiate SPDY or HTTP/2.0
connections. It is expected to be replaced by ALPN in the future. If
HTTP/2.0 is negotiated, this is indicated for the entire connection and
http.c is expected to initialize itself for HTTP/2.0 instead of
HTTP/1.1.

see:
http://technotes.googlecode.com/git/nextprotoneg.html
http://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg-04

This makes it easier to trace what's happening.

the number of elements in the 'nghttp2_session_callbacks' structure is
now reduced by 2 in version 0.3.0 (I'm not sure when the change
happened, but checking for ver 0.3.0 work for me).

Something is wrong in 'userp' for the HTTP2 recv_callback().  The
session is created using bogus user-data; '&conn' and not 'conn'.

I noticed this since the socket-value in Curl_read_plain() was set to a
impossible high value.

Fixed two compiler nits

...until the function is successful when it returns them in the out
parameters.

Added conversion functions write32_le() and write64_le() to ensure the
NTLMv2 timestamp is always written in little-endian.

It is not provided by NSS public headers.

Bug: https://bugzilla.redhat.com/1058776

... that the user does not ask for