Log Message:
ImageMagick6: Also block PS2 and PS3 coders in policy.xml

At least when reading PS2 and PS3 files via
`convert PS2:<input> <output>' and `convert PS3:<input> \ 
<output>'
gslib/ghostscript will be invoked and hence subject to VU#332928.

Pointed out by Bob Friesenhahn via oss-security@ ML (and follow up from
VU#332928 update).