Add support for DSF files to DSDIFF decoder - v4

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

doc/user.xml

@@ -782,7 +782,7 @@ systemctl start mpd.socket</programlisting>
         <title><varname>dsdiff</varname></title>
 
         <para>
-          Decodes DFF files containing DSDIFF data (e.g. SACD rips).
+          Decodes DFF and DSF files containing DSDIFF data (e.g. SACD rips).
         </para>
 
         <informaltable>