Add support for in-initramfs live images with "root=live:/path/name.img"

Add support for in-initramfs live images with "root=live:/path/name.img"

This allows creation of initramfs images which contain a Live system.
The primary use for this is keeping very large initramfs-based systems
(e.g. anaconda, the Fedora installer) compressed in-memory, by using a
compressed filesystem image like squashfs or btrfs.

dmsquash-live-genrules.sh will initqueue dmsquash-live-root itself
(rather than making udev rules) if the given live "device" is actually
an existing, plain file.

parse-dmsquash-live.sh will only accept paths that end in ".img".
dmsquash-live-root will only handle images named "*squashfs.img",
"*ext3fs.img", or "*btrfs.img".