Request body: only read body in main request (ticket #330).

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.
Events: backout eventport changes (r5172) for now.

Evenport method needs more work.  Changes in r5172, while being correct,
introduce various new regressions with current code.
nginx-1.3.16-RELEASE