hlsdemux: Implement proper segment handling

https://bugzilla.gnome.org/show_bug.cgi?id=695846
https://bugzilla.gnome.org/show_bug.cgi?id=723268
uridownload: Handle no buffers before EOS as error instead of crashing
m3u8: Fix off-by-one in the download range end
dvdspu: set proxy allocation flag on the video sink pad

https://bugzilla.gnome.org/show_bug.cgi?id=725893