Dear Wonderful Maintainer,
cdda2mp3 does not correctly handle the FILEPREFIX environment variable.  Currently, it is ignored, but the man page indicates it should work.  Current behavior is to take the command line parameter, or if unset default to a value, and overwrite the environment variable regardless.

Additionally - default values that may be set in /etc/default can currently override environment variables and command line arguments.

This patch makes the following fixes:
* Moves /etc/default handling earlier so environment and command line parameters will override defaults - as commonly expected.
* Has FILEPREFIX prefer the command line parameter, but fall back to the incoming environment variable, and if neither is set fall back to a default.