print: Match document size against known paper sizes
Custom paper sizes work with some printer drivers but not others. EPSON's CUPS driver for example converts a document of a custom-defined US Letter size to ISO A4 size (scaling down the width and scaling up the height to extend off the paper). Matching the document size against a known paper size prevents the driver from setting an erroneous paper size.
Fixes an issue discussed in comments 3, 4, 5, and 6 of https://bugs.launchpad.net/inkscape/+bug/630635.