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/cache/config.go

@@ -89,6 +89,9 @@ func parseCacheDrives(drives []string) ([]string, error) {
 	}
 
 	for _, d := range endpoints {
+		if len(d) == 0 {
+			return nil, config.ErrInvalidCacheDrivesValue(nil).Msg("cache dir cannot be an empty path")
+		}
 		if !filepath.IsAbs(d) {
 			return nil, config.ErrInvalidCacheDrivesValue(nil).Msg("cache dir should be absolute path: %s", d)
 		}