With this, `redoxfs --get-uuid <disk>` prints out the UUID of the
provided redoxfs disk image.

This is needed especially for aarch64 kernels where the UUID of the boot
disk needs to be passed to the kernel by the bootloader. With this
extension to the mount command, the kernel build glue can query the
prepared redoxfs image for it's UUID.

According to `man open`:

> The file is opened in append mode. Before each write(2), the file offset
is positioned at the end of the file, as if with lseek(2). The modi‐
fication of the file offset and the write operation are performed as a
single atomic step.