gnu: ghostscript: Fix CVE-2013-5653 and CVE-2016-{7976,7978,7979,8602}.

gnu: ghostscript: Fix CVE-2013-5653 and CVE-2016-{7976,7978,7979,8602}.

* gnu/packages/patches/ghostscript-CVE-2013-5653.patch,
gnu/packages/patches/ghostscript-CVE-2016-7976.patch,
gnu/packages/patches/ghostscript-CVE-2016-7978.patch,
gnu/packages/patches/ghostscript-CVE-2016-7979.patch,
gnu/packages/patches/ghostscript-CVE-2016-8602.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.
* gnu/packages/ghostscript.scm (ghostscript)[replacement]: New field.
(ghostscript/fixed): New variable.
(ghostscript/x): Inherit 'ghostscript/fixed'.