Upstream: p->downstream_error instead of closing connection.

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.
SSL: logging level of "peer closed connection in SSL handshake".

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.
Updated PCRE used for win32 builds.
Win32: ngx_open_dir() and ngx_close_dir() fixes.

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.