Re-add plKeyImpPublic and the patching iterators.

These were removed by Cyan in version 70, when the PageInfo version was
bumped to 6.
Reimplement PageInfo Flags

This bumps the page info version to 7 since I added a flags field, which
will, once again, be used for patch pages. I also took the liberty of removing support for prp versions before 6 (old junk) to simplify the reader.