New test case: cupsfilter.

This checks that running sample_pdf through simple_ppd gives the correct
output format.

It currently fails.
Make the sample PDF a valid one.
Fixed handling of back-channel and side-channel file descriptors.
Don't rely on the command name to know the type of process it is.
Watch for processes exiting only after they've all been started.
Perform format conversion (#15).

This just uses cupsfilter at the moment.

Now cupsfilter test-case passes.