libminixfs: fix buf leak

	. mfs: add asserts to catch similar problems in the future

Change-Id: I939230b8d63f100ac071696425b35496b2169bc4
x86_hdimage.sh -i: crossbuild x86 release CD

        . build writeisofs as a native tool too for it
        . also mkfs.mfs: make missing file in proto nonlethal
        . make setup script a little more self-sufficient
        . hdboot: use INSTALL_FILE instead of INSTALL so that the
          results get added to the METALOG

Change-Id: Id233e4c861f81046bf6c4be0510b8a3bf39ff9be