fix: detect mounts via /proc/self/mountinfo so stale FUSE works

mountinfo.Mounted lstats the path. When the sshfs link dies, every stat
on the mountpoint returns EIO, so -l filtered the dead mount out, mount
failed in MkdirAll, and -u failed before fusermount. Switch detection to
mountinfo.GetMounts (no stat) and add a fusermount -uz fallback so a
stale mount can actually be torn down.

main_test.go

@@ -200,3 +200,25 @@ func TestUnmountNotMountedExistingDir(t *testing.T) {
 		t.Fatalf("want not-mounted error, got %v", err)
 	}
 }
+
+func TestIsMountedAtFalseOnPlainDir(t *testing.T) {
+	dir := t.TempDir()
+	ok, err := is_mounted_at(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if ok {
+		t.Fatalf("plain tempdir should not be a mountpoint")
+	}
+}
+
+func TestIsMountedAtFalseOnMissingPath(t *testing.T) {
+	dir := t.TempDir()
+	ok, err := is_mounted_at(filepath.Join(dir, "nope"))
+	if err != nil {
+		t.Fatalf("unexpected error (no stat should happen): %v", err)
+	}
+	if ok {
+		t.Fatalf("missing path should not be a mountpoint")
+	}
+}