redoxfs: Add '--get-uuid' support

See merge request redox-os/redoxfs!44

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.

A little bit more documentation

See merge request redox-os/redoxfs!38

Fmaps: Fix bug with syncing

See merge request redox-os/redoxfs!36

Gitlab CI

See merge request redox-os/redoxfs!37