Update nginx from 1.17.3 to the latest bugfix release 1.17.4.

Move third-party SLICE_AHEAD knob down to other third-party
knobs. (*)

Requested by: Herbert J. Skuhra <herbert@gojira.at> (*)

<ChangeLog>

*) Change: better detection of incorrect client behavior in HTTP/2.

*) Change: in handling of not fully read client request body when

returning errors in HTTP/2.

*) Bugfix: the "worker_shutdown_timeout" directive might not work when

using HTTP/2.

*) Bugfix: a segmentation fault might occur in a worker process when

using HTTP/2 and the "proxy_request_buffering" directive.

*) Bugfix: the ECONNABORTED error log level was "crit" instead of

"error" on Windows when using SSL.

*) Bugfix: nginx ignored extra data when using chunked transfer

encoding.

*) Bugfix: nginx always returned the 500 error if the "return" directive

was used and an error occurred during reading client request body.

*) Bugfix: in memory allocation error handling.

</ChangeLog>