bug#51425: file-system: validate flags

[Jonathan Brielmaier]

Imagine the following file system definition in your config.scm:

```
(file-system
  (device (uuid "UUID-123"))
  (flags '((create-mount-point? #t)))
  (mount-point "/mnt")
  (type "ext4")))
```

When you reconfigure there will be no complain, but when you reboot your
system wont boot. The parameter to flags is nonsense, it should be
something like: `read-only`. So mounting of the file system will fail...

It would be nice if we can have some flag validation during reconfigure.

~Jonathan