Version 4 of my patch to add DSF support to the DSDIFF
decoder plugin.

This time I have taken a different approach and created a new
read_metadata function specific for reading DSF files. This saves an
indent (and for me a lot of indent nightmares) and also useful for
splitting the DSF and DFF decoders later on.

There are still a few lines which exceed the 80 character width limit by
a few chars. I was not able to stay within the limit and create (for me)
readable code.

Jurgen