Previously, nginx closed client connection in cases when a response body
from upstream was needed to be cached or stored but shouldn't be sent to
the client.  While this is normal for HTTP, it is unacceptable for SPDY.

Fix is to use instead the p->downstream_error flag to prevent nginx from
sending anything downstream.  To make this work, the event pipe code was
modified to properly cache empty responses with the flag set.

The ngx_http_upstream_dummy_handler() must be set regardless of
the read event state.  This prevents possible additional call of
ngx_http_upstream_send_request_handler().

There is no real difference on all known platforms, but it's still wrong.

Found by Coverity (CID 400876).

This allows to change the structure of cache files without spamming logs
with false alerts.

Previously, last_modified_time was tested against -1 to check if the
not modified filter should be skipped.  Notably, this prevented nginx
from additional If-Modified-Since (et al.) checks on proxied responses.
Such behaviour is suboptimal in some cases though, as checks are always
skipped on responses from a cache with ETag only (without Last-Modified),
resulting in If-None-Match being ignored in such cases.  Additionally,
it was not possible to return 412 from the If-Unmodified-Since if last
modification time was not known for some reason.

This change introduces explicit r->disable_not_modified flag instead,
which is set by ngx_http_upstream_process_headers().

See http://mailman.nginx.org/pipermail/nginx-devel/2013-November/004523.html.