Reorganize mount/unmount code so it is easier to add Darwin-specific implementation

After these changes, in order to add Darwin bind-mount implementation, one only needs:
* Adjust HasBindMounts definition in mount.go
* Provide implementation in mount_darwin.go

There was no consensus on adding dependency on bindfs, that seems to be the only working solution for bind-mounts on Darwin as of today, in https://github.com/containerd/containerd/pull/8789, that's why the actual implementation is not added in current PR.

As a bonus, Linux FUSE-related code was moved to a separate file and possibly could be reused on FreeBSD, though this needs testing.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>

pkg/os/mount_unix.go

@@ -20,11 +20,12 @@ package os
 
 import (
 	"github.com/containerd/containerd/v2/core/mount"
+	"github.com/containerd/errdefs"
 )
 
 // Mount will call unix.Mount to mount the file.
 func (RealOS) Mount(source string, target string, fstype string, flags uintptr, data string) error {
-	return mount.ErrNotImplementOnUnix
+	return errdefs.ErrNotImplemented
 }
 
 // Unmount will call Unmount to unmount the file.