Make SHOW VARIABLES output really alphabetic.

Note: the patch removes an extra declaration of
the read_only variable. It also assumes that '_'
sorts before [a-z].


mysql-test/r/fulltext_var.result:
  Fix test result to account for SHOW VARIABLES
  output reordering.
sql/set_var.cc:
  Make SHOW VARIABLES output really alphabetic.

mysql-test/r/fulltext_var.result

@@ -2,8 +2,8 @@ drop table if exists t1;
 show variables like "ft\_%";
 Variable_name	Value
 ft_boolean_syntax	+ -><()~*:""&|
-ft_min_word_len	4
 ft_max_word_len	84
+ft_min_word_len	4
 ft_query_expansion_limit	20
 ft_stopword_file	(built-in)
 create table t1 (b text not null);