Only wavpack implements both fileDecodeFunc and streamDecodeFunc, and it's
fileDecodeFunc provides more functionality.  So try using that first.

This commit also fixes a bug where the plugin test loop wouldn't break once
a suitable plugin was found if it used fileDecodeFunc.

git-svn-id: https://svn.musicpd.org/mpd/trunk@6655 09075e82-0dd4-0310-85a5-a0d7c8717e4f