Add btrfs-mksnapshot-rootfs script

Add btrfs-mksnapshot-rootfs script

This script is a wrapper for btrfs-mksnapshot script, which creates a
daily snapshot of the root filesystem (/) if it is of type "btrfs" and
when the btrfs-mksnapshot tool is available.

If the root filesystem is not of type "btrfs" or when btrfs-mksnapshot is
not available, it exits WITHOUT an error code, so it is safe to use this
wrapper even on systems which don't use btrfs at all (this is useful for
/etc/apt/apt.conf.d/06btrfs-mksnapshot, for example).