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_unsupported.go

@@ -1,4 +1,4 @@
-//go:build darwin || openbsd
+//go:build openbsd
 
 /*
    Copyright The containerd Authors.
@@ -18,29 +18,24 @@
 
 package mount
 
-import "errors"
-
-var (
-	// ErrNotImplementOnUnix is returned for methods that are not implemented
-	ErrNotImplementOnUnix = errors.New("not implemented under unix")
-)
+import "github.com/containerd/errdefs"
 
 // Mount is not implemented on this platform
 func (m *Mount) mount(target string) error {
-	return ErrNotImplementOnUnix
+	return errdefs.ErrNotImplemented
 }
 
 // Unmount is not implemented on this platform
 func Unmount(mount string, flags int) error {
-	return ErrNotImplementOnUnix
+	return errdefs.ErrNotImplemented
 }
 
 // UnmountAll is not implemented on this platform
 func UnmountAll(mount string, flags int) error {
-	return ErrNotImplementOnUnix
+	return errdefs.ErrNotImplemented
 }
 
 // UnmountRecursive is not implemented on this platform
 func UnmountRecursive(mount string, flags int) error {
-	return ErrNotImplementOnUnix
+	return errdefs.ErrNotImplemented
 }