scan: skip OS default excludes, and record nothing on filesystems that are not persistent (closes #124)

scan never records or reads anything on a filesystem that is not
persistent (tmpfs, proc, sysfs and the like), however it reaches it; no
flag turns this off. It notes each such filesystem's device ID as it
starts: on Linux from /proc/self/mountinfo, on macOS by lstat of each
listed mount point. Below an operand the walk also skips the default
excludes, a path list per system (Linux, macOS, and Windows, not used
yet); --no-default-excludes turns off only those. A default exclude that
is itself an operand gets no skip line. The tests stop at once, saying
why, when TMPDIR is not on a persistent filesystem.

Judgement call: an unreadable mount list fails the scan, and no flag gets round it.
Judgement call: a file on such a filesystem below an operand gets the same line as a directory.
Judgement call: such an operand that another operand's walk also reaches is named twice.
Unverified: whether macOS's mount list gives the device lstat reports, so macOS keeps lstat, and a hidden mount there can still add another filesystem's device.
Unverified: the macOS code is never built or run here; bind mounts need root to test.

Model: opus-5-5

exclude_darwin.go

@@ -0,0 +1,52 @@
+//go:build darwin
+
+package main
+
+import (
+	"slices"
+
+	"golang.org/x/sys/unix"
+)
+
+// nonPersistentFilesystems returns the device ID of every mounted
+// filesystem that is not persistent, with its type, from the system's
+// list of mounted filesystems. The device ID is read with lstat on the
+// mount point. Where one filesystem is mounted over another on the same
+// mount point, the later one in the list counts. A mount point that
+// lstat cannot reach is left out.
+func nonPersistentFilesystems() (map[uint64]string, error) {
+	// The first call counts the filesystems, the second lists them.
+	n, err := unix.Getfsstat(nil, unix.MNT_NOWAIT)
+	if err != nil {
+		return nil, err
+	}
+
+	list := make([]unix.Statfs_t, n)
+
+	n, err = unix.Getfsstat(list, unix.MNT_NOWAIT)
+	if err != nil {
+		return nil, err
+	}
+
+	mounts := make(map[string]string, n)
+
+	for _, st := range list[:n] {
+		mounts[unix.ByteSliceToString(st.Mntonname[:])] =
+			unix.ByteSliceToString(st.Fstypename[:])
+	}
+
+	types := nonPersistentFilesystemTypes()
+	devices := make(map[uint64]string)
+
+	for mountPoint, fsType := range mounts {
+		if !slices.Contains(types, fsType) {
+			continue
+		}
+
+		if dev, ok := deviceOfPath(mountPoint); ok {
+			devices[dev] = fsType
+		}
+	}
+
+	return devices, nil
+}