WIP: stream: implement cdda:// support through libavdevice

This implements cdda:// stream support by proxying the cdda:// URL and --cdda-*
options to libavdevice. This is only enabled if the native cdda support is
disabled (default).

So, this is just a proof-of-concept and there may be a better way to do this. I created the new stream_cdda_libav.c file to avoid cluttering stream_cdda.c with ifdefs, however there's some code overlap. Also, right now it doesn't pass any of the --cdda-* options to libavdevice, because it's not very clear to me if that's even possible given the "av://format:filename" syntax. So, yeah, if you have a better idea that'd be nice.