Add constants for commonly used values. (#3588)

This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.

This also fixes `goconst ./...` reported issues.

cmd/update-notifier.go

@@ -66,7 +66,7 @@ func colorizeUpdateMessage(updateString string, newerThan time.Duration) string
 	default:
 		// on windows terminal turn off unicode characters.
 		var top, bottom, sideBar string
-		if runtime.GOOS == "windows" {
+		if runtime.GOOS == globalWindowsOSName {
 			top = yellow("*" + strings.Repeat("*", maxContentWidth) + "*")
 			bottom = yellow("*" + strings.Repeat("*", maxContentWidth) + "*")
 			sideBar = yellow("|")