Added util.ByteSizeNoLimit: a byte size of -1 means no limit, and any other negative size is rejected

ParseByteSize accepts "-1" or "-1B" as ByteSizeNoLimit, and String gives it
back as "-1". A setting for which no limit is unsafe can reject it.

internal/util/bytesize_test.go

@@ -1,6 +1,9 @@
 package util
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestByteSizeString(t *testing.T) {
 	tests := []struct {
@@ -25,3 +28,23 @@ func TestByteSizeString(t *testing.T) {
 		}
 	}
 }
+
+// -1 is ByteSizeNoLimit, parsed and formatted as "-1"; no other size may be
+// negative.
+func TestByteSizeNoLimit(t *testing.T) {
+	for _, text := range []string{"-1", "-1B", "-1b", " -1 "} {
+		size, err := ParseByteSize(text)
+		if err != nil || size != ByteSizeNoLimit {
+			t.Errorf("ParseByteSize(%q) = %d, %v; want ByteSizeNoLimit", text, size, err)
+		}
+	}
+	if got := ByteSizeNoLimit.String(); got != "-1" {
+		t.Errorf("ByteSizeNoLimit.String() = %q; want -1", got)
+	}
+	for _, text := range []string{"-2", "-1KB", "-0.5", "-1KiB", "-100"} {
+		size, err := ParseByteSize(text)
+		if err == nil || !strings.Contains(err.Error(), "only -1 (no limit)") {
+			t.Errorf("ParseByteSize(%q) = %d, %v; want a negative size error", text, size, err)
+		}
+	}
+}