lp:702084 - myisam_block_size is not reported in SHOW GLOBAL VARIABLES

add a read-only server variable @@myisam_block_size

mysql-test/r/myisam.result

@@ -2445,3 +2445,9 @@ DROP TABLE t1;
 SET myisam_sort_buffer_size=@@global.myisam_sort_buffer_size;
 SET myisam_repair_threads=@@global.myisam_repair_threads;
 End of 5.1 tests
+show variables like 'myisam_block_size';
+Variable_name	Value
+myisam_block_size	1024
+select @@global.myisam_block_size;
+@@global.myisam_block_size
+1024

mysql-test/t/myisam.test

@@ -1693,3 +1693,6 @@ SET myisam_sort_buffer_size=@@global.myisam_sort_buffer_size;
 SET myisam_repair_threads=@@global.myisam_repair_threads;
 
 --echo End of 5.1 tests
+
+show variables like 'myisam_block_size';
+select @@global.myisam_block_size;

sql/set_var.cc

@@ -469,6 +469,10 @@ static sys_var_thd_enum         sys_myisam_stats_method(&vars, "myisam_stats_met
                                                 &myisam_stats_method_typelib,
                                                 NULL);
 
+static sys_var_const            sys_myisam_block_size(&vars, "myisam_block_size",
+                                             OPT_GLOBAL, SHOW_LONG,
+                                             (uchar*) &myisam_block_size);
+
 #ifdef __NT__
 /* purecov: begin inspected */
 static sys_var_const            sys_named_pipe(&vars, "named_pipe",