00-livecd.sh: ${LOOPEXT} is never set, drop it
00-livecd.sh: drop old TODO
00-livecd.sh: fix "docache" support, unfuck the whole code

docache support was broken since the Stone age. Not all the
loop types correctly supported it because it wasn't transparent
to them and required manual hackery. Now _setup_livecd_content
transparently configures LOOP_PATH if docache is enabled.