Improvement of the auxiliary routines include/show_msg*.inc

- replace @message by $message
  --> The annoying protocol line "SET @message= <value>" will disappear.
- extended description
- some examples added

mysql-test/include/show_msg.inc

@@ -6,13 +6,19 @@
 # Usage:
 #    Add the following to any *.test file:
 #      :
-#    set @message="This is a message example";
+#    let $message= <value>;
 #    --source include/show_msg.inc
 #      :
 #
+# Attention:
+#   - Please do not write any spaces between $message and the "=", because the
+#     assignment will not work.
+#   - Be careful with single quotes. They must be escaped like "''" or "\'".
+#
+# "include/show_msg80.inc" contains a detailed description and examples.
 
 --disable_query_log
-SET @utf8_message = CONVERT(@message using utf8);
+eval SET @utf8_message = CONVERT('$message' using utf8);
 select @utf8_message as ""
 union
 select repeat(CONVERT('-' using utf8),char_length(@utf8_message));