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/fuse_linux.go

@@ -0,0 +1,50 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package mount
+
+import (
+	"os/exec"
+
+	"golang.org/x/sys/unix"
+)
+
+// fuseSuperMagic is defined in statfs(2)
+const fuseSuperMagic = 0x65735546
+
+func isFUSE(dir string) bool {
+	var st unix.Statfs_t
+	if err := unix.Statfs(dir, &st); err != nil {
+		return false
+	}
+	return st.Type == fuseSuperMagic
+}
+
+// unmountFUSE attempts to unmount using fusermount/fusermount3 helper binary.
+//
+// For FUSE mounts, using these helper binaries is preferred, see:
+// https://github.com/containerd/containerd/pull/3765#discussion_r342083514
+func unmountFUSE(target string) error {
+	var err error
+	for _, helperBinary := range []string{"fusermount3", "fusermount"} {
+		cmd := exec.Command(helperBinary, "-u", target)
+		err = cmd.Run()
+		if err == nil {
+			return nil
+		}
+	}
+	return err
+}