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.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.
@@ -157,7 +157,7 @@ func getDiskInfo(diskPath string) (di disk.Info, err error) {
 func (s *posix) checkDiskFree() (err error) {
 	// We don't validate disk space or inode utilization on windows.
 	// Each windows calls to 'GetVolumeInformationW' takes around 3-5seconds.
-	if runtime.GOOS == "windows" {
+	if runtime.GOOS == globalWindowsOSName {
 		return nil
 	}