gfs2_tool: Update /etc/mtab with metafs mounts, handle interrupts
    
    This patch gives libgfs2 the ability to add the metafs to the
    /etc/mtab file, so that a "mount" will show it and "umount -a -tgfs2"
    will unmount it. At the same time, it also catches interrupt signals
    while gfs2_tool df is running, and if interrupted, it unmounts the
    metafs.
    
    rhbz#996233