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

scan never records anything on a filesystem that is not persistent
(tmpfs, proc, sysfs and the other kernel or memory-backed types). As it
starts it notes the device ID of each such mount point, from
/proc/self/mountinfo on Linux and the mounted-filesystem list on macOS,
and checks every operand, directory and file the walk meets against
them; no flag turns this off. Below an operand the walk also skips the
default excludes, a path list per system (Linux, macOS, and Windows,
which is not used yet), and --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: an operand on such a filesystem that another operand's walk also reaches is named twice.
Unverified: the macOS mount list is built for macOS but never run; bind mounts need root to test.

Model: opus-5-5

exclude_darwin.go

@@ -0,0 +1,32 @@
+//go:build darwin
+
+package main
+
+import "golang.org/x/sys/unix"
+
+// mountedFilesystems returns every mount point with the type of its
+// filesystem, from the system's list of mounted filesystems. Where one
+// filesystem is mounted over another, the later one in the list counts.
+func mountedFilesystems() (map[string]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[:])
+	}
+
+	return mounts, nil
+}