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.

Reported by Piotr Sikora.

To avoid further breaks it's now done properly, all the dependencies
are now passed to Makefile.PL.  While here, fixed include list passed to
Makefile.PL to use Makefile variables rather than a list expanded during
configure.

It will be called in ngx_http_upstream_finalize_request().

Specifying zero rate caused division by zero when calculating delays.

Previously, we sometimes passed constant value 1 that happens to
match PCRE_CASELESS and thus was harmless.

It's no longer necessary to update src/http/modules/perl/nginx.pm
when version is bumped, as it's now derived from src/core/nginx.h.