nghttpx does not perform upgrade if the request has request body.

With --spdy-bridge option, it listens SPDY/HTTPS connections from
front end and forwards them to the backend in SPDY. The usage will be
written later. This change fixes the crash when more than 2
outstanding SpdyDownstreamConnection objects are added to SpdySession
and establishing connection to SPDY backend is failed.

In client mode, now SPDY connection to the backend server is
established per thread.  The frontend connections which belong to the
same thread share the SPDY connection.

With --client-mode option, shrpx now accepts unencrypted HTTP
connections and communicates with backend server in SPDY.  In short,
this is the "reversed" operation mode against normal mode.  This may
be useful for testing purpose because it can sit between HTTP client
and shrpx "normal" mode.

To distinguish the to-be-installed programs and non-installable
example source code, the former programs, spdycat, spdydyd and shrpx,
were moved to src directory. spdynative was removed from Makefile
because it does not appeal to any users much.

Put libhtparse in examples/htparse