Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

cmd/config-migrate.go

@@ -445,7 +445,7 @@ func migrateV5ToV6() error {
 
 	if cv5.Logger.ElasticSearch.URL != "" {
 		var url *xnet.URL
-		url, err = xnet.ParseURL(cv5.Logger.ElasticSearch.URL)
+		url, err = xnet.ParseHTTPURL(cv5.Logger.ElasticSearch.URL)
 		if err != nil {
 			return err
 		}