gnu: Add audiofile.

gnu: Add audiofile.

Patches should fix all CVEs reported by `guix lint`:
CVE-2015-7747; CVE-2017-6827, CVE-2017-6828, CVE-2017-6829,
CVE-2017-6830, CVE-2017-6831, CVE-2017-6832, CVE-2017-6833,
CVE-2017-6834, CVE-2017-6835, CVE-2017-6836, CVE-2017-6837,
CVE-2017-6838, CVE-2017-6839; CVE-2018-13440; CVE-2018-17095

Since the patches do not reference to CVEs, it's a bit hard to tell which
patch actually closes which CVE.  Debian reports all these to be closed by
the patches below and NixPkgs provides references.

* gnu/packages/audio.scm (audiofile): New variable.
* gnu/packages/patches/audiofile-fix-datatypes-in-tests.patch,
  gnu/packages/patches/audiofile-fix-sign-conversion.patch,
  gnu/packages/patches/audiofile-CVE-2015-7747.patch,
  gnu/packages/patches/audiofile-CVE-2018-13440.patch,
  gnu/packages/patches/audiofile-CVE-2018-17095.patch,
  gnu/packages/patches/audiofile-Check-the-number-of-coefficients.patch,
  gnu/packages/patches/audiofile-Fail-on-error-in-parseFormat.patch,
  gnu/packages/patches/audiofile-Fix-index-overflow-in-IMA.cpp.patch,
  gnu/packages/patches/audiofile-Fix-multiply-overflow-sfconvert.patch,
  gnu/packages/patches/audiofile-Fix-overflow-in-MSADPCM-decodeSam.patch,
  gnu/packages/patches/audiofile-division-by-zero-BlockCodec-runPull.patch,
  gnu/packages/patches/audiofile-hurd.patch,
  gnu/packages/patches/audiofile-signature-of-multiplyCheckOverflow.patch:
  New files.
* gnu/local.mk: Add them.