upstream_response logs more detailed information.

Color severity level if stderr refers to a terminal.

Added macros which log messages from the following components are
prefixed with their component name + object pointer address:

ListenHandler: LISTEN
ThreadEventReceiver: THREAD_RECV
Upstream: UPSTREAM
Downstream: DOWNSTREAM
DownstreamConnection: DCONN
SpdySession: DSPDY

This avoids the need to provide the password for your
private key interactively.

It can be used via --private-key-passwd-file or private-key-passwd-file
in the given config file. The first line in the file
(without \n) will be treated as the passwd. There isn't
any validation and all lines after the first one (if any)
are ignored.

The security model behind this is a bit simplistic so I
am open to better ideas. Basically your password file
should be root:root (700) and you *should* drop root
and run as an unprivileged user.

If the file exists and a line can be read then a callback
will be set for the SSL ctxt and it'll feed the passwd
when the private key is read (if password is needed).

If the file exists with the wrong permisions it'll be
logged and ignored.

Use --no-tls option to disable SSL/TLS and specify SPDY protocol
version using -2 or -3.

The -k, --insecure option is added to skip this verification.  The
system wide trusted CA certificates will be loaded at startup. The
--cacert option is added to specify the trusted CA certificate file.

Possibly because of deferred callback, we may get this callback when
the output buffer is not empty.

With --client-proxy option, shrpx makes sure that the request path is
an absolute URI, otherwise it will return 400 status code.

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.

Don't call HttpsUpstream::resume_read() from the call tree of
on_read().  Avoid parsing next http data after parse error.

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.