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/posix-prepare-path.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ * Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,7 +24,7 @@ import (
 
 // preparePath rewrites path to handle any OS specific details.
 func preparePath(path string) string {
-	if runtime.GOOS == "windows" {
+	if runtime.GOOS == globalWindowsOSName {
 		// Microsoft Windows supports long path names using
 		// uniform naming convention (UNC).
 		return UNCPath(path)