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>

core/mount/mount.go

@@ -19,12 +19,16 @@ package mount
 import (
 	"fmt"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/containerd/containerd/api/types"
 	"github.com/containerd/continuity/fs"
 )
 
+// HasBindMounts This is a flag to conditionally disable code that relies on working bind-mount support, so such code is easier to find across codebase.
+const HasBindMounts = runtime.GOOS != "darwin" && runtime.GOOS != "openbsd"
+
 // Mount is the lingua franca of containerd. A mount represents a
 // serialized mount syscall. Components either emit or consume mounts.
 type Mount struct {