This commit also fixes the rare issue that connection is not made
properly because of race between nghttpx process and TCP client
connection.

This reverts commit c42296ac.

Now we define NGHTTP2_SSIZE_T which is typedef-ed to the appropriate
type depending on the platform (x86/x86_64).

See GH-616 for details

This commit adds ALPN support to tutorial client/server.  It also adds
a code to check h2 was negotiated, if not, drop connection.

For tutorial server, now it sends connection preface just after TLS
handshake was made without waiting for the client connection preface.

We have to gracefully handle the case where response ends before
request body is fully received.

Some API processing is very slow (e.g., getaddrinfo).  To avoid to
slow down regular request handling, if multi threaded configuration is
enabled, we allocate dedicated worker for API.

This function sets the maximum length of header block (a set of header
fields per HEADERS frame) to send.  The length of given set of header
fields is calculated using nghttp2_hd_deflate_bound().  Previously,
this is hard-coded, and is 64KiB.

h2load has supported uploading a file quite a while, but it turns out
that it worked with HTTP/2 and SPDY only.  HTTP/1 with upload did not
work.  This commit fixes this bug, and implement HTTP/1 upload.  Due
to architectural limitation of h2load, when -d option is used, the
number of in-flight pipe-lined requests is set to 1.