backend: Use Message<_, BorrowedFd> rather than RawFd

[ids1024]

This is a breaking change to wayland-backend, but not to wayland-client or wayland-server. I believe that should be fine.

This avoids an unsafe conversion in the rs backend.

Based on #677.

[elinorbgr]

Make a breaking change in wayland-backend will force a following breaking release in wayland-client and wayland-server anyway, but I think this is a direction we want to go.

[ids1024]

Oh right, those crates expose the Backend type, so it's a public dependency. Otherwise such a change would be non-breaking.

This change can be left until the next time wayland-rs has a breaking change. This isn't urgent since it isn't actually a soundness issue and shouldn't impact behavior. (And indeed, the fact this change works confirms that the RawFds are already being used correctly.)