Revert "Drop numbers.{c,h}"

This reverts commit commit 959eebda40227aa05c731846a0142d9b09c1c3d8
Revert "Drop mfileio.{c,h}"

This reverts commit 6af19c85a93f85165261397fb335b6cff4e9867a
Move DVIPDFMx derived files to own directory
Use default JPEG image resolution if 72.0

If we can’t retrieve the resolution from the image, we will end up
dividing by zero (the initial value we use).

Fixes https://sourceforge.net/p/xetex/bugs/71/