To prevent infinite loop in readwrite_data() function when stream is
reset before any response body comes, reset closed flag to false once
it is evaluated to true.

"Expect: 100-continue", which was once deprecated in HTTP/2, is now
resurrected in HTTP/2 draft 14.  This change adds its support to
HTTP/2 code.  This change also includes stricter header field
checking.

This is now used by the http2 code. It has two different symbols at the
end of the base64 table to make the output "url safe".

Bug: https://github.com/tatsuhiro-t/nghttp2/issues/62

1 - fixes the warnings when built without http2 support

2 - adds CURLE_HTTP2, a new error code for errors detected by nghttp2
basically when they are about http2 specific things.

This regression was introduced when *init was split into *init and
*setup...

Http2 connections would wrongly get closed after each individual
request.

Co-authored-by: Tatsuhiro Tsujikawa
Bug: http://curl.haxx.se/bug/view.cgi?id=1374

Now nghttp2_submit_request returns assigned stream ID, we don't have
to check stream ID using before_stream_send_callback.  The
adjust_priority_callback was removed.

commit 6d5f40238028f2d8c (Apr 27) or later nghttp2 is now required

This makes the findprotocol() function work as intended so that libcurl
can properly be restricted to not support HTTP while still supporting
HTTPS - since the HTTPS handler previously set both the HTTP and HTTPS
bits in the protocol field.

This fixes --proto and --proto-redir for most SSL protocols.

This is done by adding a few new convenience defines that groups HTTP
and HTTPS, FTP and FTPS etc that should then be used when the code wants
to check for both protocols at once. PROTO_FAMILY_[protocol] style.

Bug: https://github.com/bagder/curl/pull/97
Reported-by: drizzt

... and use Curl_safefree() instead of free()

For HTTP/2, we may read up everything including responde body with
header fields in Curl_http_readwrite_headers. If no content-length is
provided, curl waits for the connection close, which we emulate it
using conn->proto.httpc.closed = TRUE. The thing is if we read
everything, then http2_recv won't be called and we cannot signal the
HTTP/2 stream has closed. As a workaround, we return nonzero from
data_pending to call http2_recv.

nghttp2 has yet again extended its callback struct and this is an
attempt to make curl compile with nghttp2 from current git

This patch enables HTTP POST/PUT in HTTP2.
We disabled Expect header field and  chunked transfer encoding
since HTTP2 forbids them.
In HTTP1, Curl sends small upload data with request headers, but
HTTP2 requires upload data must be in DATA frame separately.
So we added some conditionals to achieve this.

A server might respond with a content-encoding header and a response
that was encoded accordingly in HTTP-draft-09/2.0 mode, even if the
client did not send an accept-encoding header earlier. The server might
not send a content-encoding header if the identity encoding was used to
encode the response.

See:
http://tools.ietf.org/html/draft-ietf-httpbis-http2-09#section-9.3

As this is mandated by the http2 spec draft-09

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.

... 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

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).