ao_jack: allow more control about channel layouts
core: make network options available even if old net code is disabled

Preparation for removing the old network code.
Remove internal network support

This commit removes the "old" networking code in favor of libavformat's
code.

The code was still used for mp_http, udp, ftp, cddb. http has been
mapped to libavformat's http support since approximately 6 months ago.
udp and ftp have support in ffmpeg (though ftp was added only last
month). cddb support is removed with this commit - it's probably not
important and rarely used if at all, so we don't care about it.