HTTP2: add layer between existing http and socket(TLS) layer

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.

lib/http.h

@@ -149,6 +149,9 @@ struct HTTP {
                         points to an allocated send_buffer struct */
 };
 
+typedef int (*sending)(void); /* Curl_send */
+typedef int (*recving)(void); /* Curl_recv */
+
 struct http_conn {
 #ifdef USE_NGHTTP2
 #define H2_BINSETTINGS_LEN 80
@@ -158,6 +161,9 @@ struct http_conn {
   char *mem;     /* points to a buffer in memory to store or read from */
   size_t len;    /* size of the buffer 'mem' points to */
   bool bodystarted;
+  sending send_underlying; /* underlying send Curl_send callback */
+  recving recv_underlying; /* underlying recv Curl_recv callback */
+  bool closed; /* TRUE on HTTP2 stream close */
 #else
   int unused; /* prevent a compiler warning */
 #endif