gnu: icecat: Add fixes for CVE-2015-{4473,4482,4488,4489,4491,4492}.

WARNING: CVE-2015-4473 may not be fully addressed here, because I was unable
to backport some of the patches (for upstream bugs 1182711 and 1146213).  I
was also unable to backport CVE-2015-4484 (upstream bug 1171540) and
CVE-2015-4487 (upstream bug 1171603).  I was unable to find any commit in the
upstream repository that claims to address bug 1105914 (CVE-2015-4478).

* gnu/packages/patches/icecat-CVE-2015-4473-partial.patch,
  gnu/packages/patches/icecat-CVE-2015-4482.patch,
  gnu/packages/patches/icecat-CVE-2015-4488.patch,
  gnu/packages/patches/icecat-CVE-2015-4489.patch,
  gnu/packages/patches/icecat-CVE-2015-4491.patch,
  gnu/packages/patches/icecat-CVE-2015-4492.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/gnuzilla.scm (icecat)[source]: Add patches.