Fixes for djvu2pdf

[rbrito]

Hi, @vindvaki.

I made some changes to djvu2pdf that I believe that improve both its performance as well as its usability (like being able to invoke it outside of the directory where it is located).

The commits that I'm sending here are very fine grained exactly to ease the review process as well as for "git-bisectability" (and also because I like independent changes to be in different commits).

There are other things to fix, like some pathological TOCs having "page numbers" rejected by pdfbeads (something like #p0004.djvu and so on).

I will fix those and send more pull requests if you wish/accept those.

Thanks for the original tool,

Rogério Brito.