Change to Parallel::Forkmanager

The old module "Proc::ParallelLoop" for multi-CPU processing seems to be
abandoned since 2003.

Using well maintained and documented "Parallel::Forkmanager" instead
solves two problems:
1. Bug in Proc::ParallelLoop casues crash for asymmeteric loads. I have
actually experienced this bug during transcoding a huge flac file in
parallel with tanscoding many smaller flac files.
2. Parallel::Forkmanager works under MS Win (Proc::ParallelLoop doesn't)

Checking for operating system is no longer necessary.