gnu: icecat: Add several security fixes.

* gnu/packages/patches/icecat-CVE-2015-4477.patch,
gnu/packages/patches/icecat-CVE-2015-7207.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt01.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt02.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt03.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt04.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt05.patch,
gnu/packages/patches/icecat-CVE-2016-1952-pt06.patch,
gnu/packages/patches/icecat-CVE-2016-1954.patch,
gnu/packages/patches/icecat-CVE-2016-1960.patch,
gnu/packages/patches/icecat-CVE-2016-1961.patch,
gnu/packages/patches/icecat-CVE-2016-1962.patch,
gnu/packages/patches/icecat-CVE-2016-1964.patch,
gnu/packages/patches/icecat-CVE-2016-1965.patch,
gnu/packages/patches/icecat-CVE-2016-1966.patch,
gnu/packages/patches/icecat-CVE-2016-1974.patch,
gnu/packages/patches/icecat-bug-1248851.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/gnuzilla.scm (icecat)[source]: Add patches.