scan: skip OS-appropriate default excludes (/proc, /sys, /tmp, /run, /var/tmp on Linux) (closes #124)

Below an operand, the walk no longer enters the default excludes: on
Linux /proc, /sys, /dev, /run, /tmp, /var/tmp and every mount point of a
kernel or memory-backed filesystem, read once from /proc/self/mountinfo;
on macOS /dev, /private/tmp, /private/var/tmp, /private/var/vm,
/System/Volumes/VM and /System/Volumes/Data. The lists live in the new
exclude.go. An operand at or inside one is still walked, also under
another operand, as with -x. Each skipped directory gets one stderr line
through the warning path, is not counted as skipped, and its records
count as outside the scanned operands. --no-default-excludes turns them
off; scan --help lists them.

Judgement call: an unreadable /proc/self/mountinfo fails the scan.
Judgement call: where filesystems are mounted over one another, the last decides.

Model: opus-5-5

main_test.go

@@ -535,7 +535,7 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	}{
 		cmdScan: {
 			run: func() error {
-				return runScan(t.Context(), []string{missing}, 1, false)
+				return runScan(t.Context(), []string{missing}, 1, false, false)
 			},
 			want: scanWant,
 		},