Print help that was asked for to stdout (closes #179)

--help, -h and the help subcommand now print to stdout and exit 0, so
the help can be paged and searched. The usage text of a usage error
stays on stderr with exit 2, as does help for an unknown subcommand
("sfdupes help scna"). A failed write of the help exits 1.

Cobra prints help to the command's output writer, which stays stderr,
drops a failed write, and returns no error after help. So run replaces
the help function and the help subcommand: the help goes into a buffer,
then to stdout, and the error sets the exit code.

README "Settings" and goal 4 say so.

Model: opus-5-5

README.md

@@ -253,9 +253,9 @@ In order:
    `trees` check only that each copy they name still exists, and leave out a
    copy that is gone. `scan` is built to run from `cron`. The reports run at any
    time, against what the database holds.
-4. **Keep the streams apart.** stdout holds only what was asked for:
+4. **Keep the streams apart.** Everything on stdout is machine-readable data.
-   machine-readable data, or help. Everything else for a human reader, such as
+   Everything for a human reader, such as progress and warnings, goes to stderr.
-   progress and warnings, goes to stderr.
+   The one exception is help that was asked for, which goes to stdout.
 
 ### Subcommands
 

internal/sfdupes/cli_test.go

@@ -773,36 +773,6 @@ func TestRunHelp(t *testing.T) {
 	}
 }
 
-func TestRunHelpScanPrintsScanHelp(t *testing.T) {
-	t.Parallel()
-
-	// "sfdupes help scan" and "sfdupes --help scan" print exactly what
-	// "sfdupes scan --help" prints: the help of scan, not of sfdupes.
-	var want bytes.Buffer
-
-	run([]string{cmdScan, helpFlag}, &want, io.Discard)
-
-	for _, args := range [][]string{
-		{helpCommand, cmdScan}, {helpFlag, cmdScan},
-	} {
-		var stdout, stderr bytes.Buffer
-
-		code := run(args, &stdout, &stderr)
-		if code != exitOK {
-			t.Errorf("run(%v) = %d, want %d", args, code, exitOK)
-		}
-
-		if got := stdout.String(); got != want.String() {
-			t.Errorf("run(%v) stdout = %q, want what scan %s prints, %q",
-				args, got, helpFlag, want.String())
-		}
-
-		if got := stderr.String(); got != "" {
-			t.Errorf("run(%v) stderr = %q, want nothing", args, got)
-		}
-	}
-}
-
 func TestRunScanUsageLine(t *testing.T) {
 	t.Parallel()