This is to avoid setting the TCP_NODELAY flag on SPDY socket in
ngx_http_upstream_send_response().  The latter works per request,
but in SPDY case it might affect other streams in connection.

As of 1.3.9, chunked request body may be available with
r->headers_in.content_length_n <= 0.  Additionally, request body
may be in multiple buffers even if r->request_body_in_single_buf
was requested.

Dependancy tracking introduced in r5169 were not handled absolute path
names properly.  Absolute names might appear in CORE_DEPS if --with-openssl
or --with-pcre configure arguments are used to build OpenSSL/PCRE
libraries.

Additionally, revert part of r5169 to set NGX_INCS from Makefile
variables.  Makefile variables have $ngx_include_opt in them, which
might result in wrong include paths being used.  As a side effect,
this also restores build with --with-http_perl_module and --without-http
at the same time.

Evenport method needs more work.  Changes in r5172, while being correct,
introduce various new regressions with current code.

Before 1.3.9 an attempt to read body in a subrequest only caused problems
if body wasn't already read or discarded in a main request.  Starting with
1.3.9 it might also cause problems if body was discarded by a main request
before subrequest start.

Fix is to just ignore attempts to read request body in a subrequest, which
looks like right thing to do anyway.

We generate both read and write events if an error event was returned by
port_getn() without POLLIN/POLLOUT, but we should not try to handle inactive
events, they may even have no handler.