The ngx_open_dir() function changed to restore name passed to it.  This
fixes removing destination directory in dav module, as caught by dav.t.
The ngx_close_dir() function introduced to properly convert errors, as
FindClose() returns 0 on error.

Previously, the NGX_LOG_INFO level was used unconditionally.  This is
correct for client SSL connections, but too low for connections to
upstream servers.  To resolve this, ngx_connection_error() now used
to log this error, it will select logging level appropriately.

With this change, if an upstream connection is closed during SSL
handshake, it is now properly logged at "error" level.

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().

The check became meaningless after refactoring in 2a92804f4109.
With the loop currently in place, "current" can't be NULL, hence
the check can be dropped.

Additionally, the local variable "current" was removed to
simplify code, and pool->current now used directly instead.

Found by Coverity (CID 714236).

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

Found by Coverity (CID 400876).

This isn't really important as configuration testing shortly ends with
a process termination which will free all sockets, though Coverity
complains.

Prodded by Coverity (CID 400872).