compose: Support 'remove-from-packages' entry

This is the equivalent of the 'removefrom' verb in Lorax's templating.
It's a lot more robust than a generic "rm-rf" type thing, because most
often you only want to remove files from particular packages.
compose: Support 'postprocess-script'

This is obviously a total cop-out.  However, without glibc fixes, we
can't do better.  See:

https://bugzilla.redhat.com/show_bug.cgi?id=156477