If sub_filter directive was only specified at http{} level, sub filter
internal data remained uninitialized.  That would lead to a crash in runtime.

The SPDY support is removed, as it's incompatible with the new module.

A configuration with a named location inside a zero-length prefix
or regex location used to trigger a segmentation fault, as
ngx_http_core_location() failed to properly detect if a nested location
was created.  Example configuration to reproduce the problem:

    location "" {
        location @foo {}
    }

Fix is to not rely on a parent location name length, but rather check
command type we are currently parsing.

Identical fix is also applied to ngx_http_rewrite_if(), which used to
incorrectly assume the "if" directive is on server{} level in such
locations.

Reported by Markus Linnala.
Found with afl-fuzz.

When prototyping behavior is not explicitly specified, xsubpp emits
a message to stderr asking to do so (see ticket #608).

Resolved warnings about declarations that hide previous local declarations.
Warnings about WSASocketA() being deprecated resolved by explicit use of
WSASocketW() instead of WSASocket().  When compiling without IPv6 support,
WinSock deprecated warnings are disabled to allow use of gethostbyname().

Iterating through all connections takes a lot of CPU time, especially
with large number of worker connections configured.  As a result
nginx processes used to consume CPU time during graceful shutdown.
To mitigate this we now only do a full scan for idle connections when
shutdown signal is received.

Transitions of connections to idle ones are now expected to be
avoided if the ngx_exiting flag is set.  The upstream keepalive module
was modified to follow this.