Based on the XSPF playlist plugin.

This plugin will replace the last.fm input plugin, once the playlist
API is integrated into MPD.

This patch completes the configuration support.

Prevent access on uninitialized variable if the plugin list is empty.

Based on this API, we will add parsers for EXTM3U, PLS, ASX, last.fm
radio and others.

There is no integration into the MPD core yet.  Right now, we have a
command line test program.  This is work in progress.