[FFe] Please update nginx to 1.9.12

NGINX has recently uploaded version 1.9.12. This contains bugfixes and feature changes to core functionality in nginx, as well as introduces workarounds due to some issues with OpenSSL.

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The upstream changelog is as follows. Comments of mine will be added with hashtags (#):

Changes with nginx 1.9.12 24 Feb 2016

    *) Feature: Huffman encoding of response headers in HTTP/2.
       Thanks to Vlad Krasnov.

    *) Feature: the "worker_cpu_affinity" directive now supports more than
       64 CPUs.

    *) Bugfix: compatibility with 3rd party C++ modules; the bug had
       appeared in 1.9.11.
       Thanks to Piotr Sikora.

    *) Bugfix: nginx could not be built statically with OpenSSL on Linux;
       the bug had appeared in 1.9.11.

    *) Bugfix: the "add_header ... always" directive with an empty value did
       not delete "Last-Modified" and "ETag" header lines from error
       responses.

    *) Workaround: "called a function you should not call" and "shutdown
       while in init" messages might appear in logs when using OpenSSL
       1.0.2f.

    *) Bugfix: invalid headers might be logged incorrectly.

    *) Bugfix: socket leak when using HTTP/2.

    *) Bugfix: in the ngx_http_v2_module.

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Rationale for Update:

A longer term goal of the Server Team is to have NGINX 1.10.x (the next stable release of NGINX) in Ubuntu Xenial, either as a post-Xenial-release SRU, or right before we release, depending on when NGINX releases the 1.10.x branch (which is based off the 1.9.x branch).

To reduce the delta between merges, and to reduce the amount of work necessary to switch from 1.9.x to 1.10.x in the future of Xenial, it is more preferable to have the latest 1.9.x available as close to 1.10.x release as possible; this makes it much more easier to deal with build failures, or to troubleshoot issues between versions, as there is less of an upstream delta.

We do not enable HTTP/2, therefore the only feature change we should be worried about here is the ability of the `worker_cpu_affinity` directive being able to support more than 64 CPUs. This does not affect other functionality.

We have other bugfixes, as well, for other functions in the system, including 'add_header' directives, and incorrect logging of invalid headers. These do not affect any of the other features in Ubuntu, nor does this appear to affect the feature-set of NGINX.

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Security Considerations:

HTTP/2
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* HTTP/2 is disabled in the NGINX builds currently. This is at the request of the Security Team. There is no security concern of HTTP/2 in this release.

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Files to be uploaded (Pending creation)

* Debdiff between 1.9.11-0ubuntu2 and 1.9.12-0ubuntu1 (direct upload)
* Local sbuild logs (amd64, i386), including 'lintian' checks
* Installation log (Xenial amd64 VM)