Log Message:
ImageMagick6: reflect security policy options

As noted by wiz@, since this package now offers different security
policy configuration files, we should reflect this choice, as we already
do with ImageMagick(7). To borrow from wiz@'s earlier commit message:

Add options for the security policies. Follow upstream for the default.
This can be changed after installation by copying the appropriate config
file to etc/ImageMagick-6/policy.xml.