This is because _LDFLAGS comes before _LDADD.  If we specify a library
and another library in _LDADD depends on it, we get undefined
reference error.

Same reported at #87 but at src/shrpx_config.cc src/instead of util.cc

We may later add append(const T &), where we require T for .size() and
.data().  We can coexist former append_cstr and new one.  If we pass
string literal, append(const (&)[N]) is selected.

nghttp2_submit_shutdown_notice() is used to notify the client that
graceful shutdown is started.  We expect that after this call, the
server application should send another GOAWAY using
nghttp2_submit_goaway() with appropriate last_stream_id.  In this
commit, we also added nghttp2_session_get_last_proc_stream_id(), which
can be used as last_stream_id parameter.

This commit implements graceful shutdown in nghttpx.  The integration
test for graceful shutdown is also added.

SSL_write may return error indicating blocking until all given data
are written.  Because of this, it is preferable to update
last_write_time_ after SSL_write regardless of its return value.

There is no application level failure in h1 upstream.  For h2, SPDY
upstreams, don't call DIE(), instead return -1 to delete handler.