tool_parsecfg: make warning output propose double-quoting

When the config file parser detects a word that *probably* should be
quoted, mention double-quotes as a possible remedy.

Test 459 verifies.

Proposed-by: Jiehong on github
Fixes #12409
Closes #12412

src/tool_parsecfg.c

@@ -210,7 +210,8 @@ int parseconfig(const char *filename, struct GlobalConfig *global)
             break;
           default:
             warnf(operation->global, "%s:%d: warning: '%s' uses unquoted "
-                  "whitespace in the line that may cause side-effects",
+                  "whitespace that may cause side-effects. Consider quoting "
+                  "the value with double quotes?",
                   filename, lineno, option);
           }
         }