Sys_var_plugin: use const for default plugin name

sql/mysqld.cc

@@ -348,8 +348,8 @@ static char *lc_messages;
 static char *lc_time_names_name;
 char *my_bind_addr_str;
 static char *default_collation_name;
-char *default_storage_engine, *default_tmp_storage_engine;
-char *enforced_storage_engine=NULL;
+const char *default_storage_engine, *default_tmp_storage_engine;
+const char *enforced_storage_engine=NULL;
 char *gtid_pos_auto_engines;
 plugin_ref *opt_gtid_pos_auto_plugins;
 static char compiled_default_collation_name[]= MYSQL_DEFAULT_COLLATION_NAME;
@@ -4854,7 +4854,8 @@ static void add_file_to_crash_report(char *file)
   init_default_storage_engine_impl(#X, X, &global_system_variables.Y)
 
 static int init_default_storage_engine_impl(const char *opt_name,
-                                            char *engine_name, plugin_ref *res)
+                                            const char *engine_name,
+                                            plugin_ref *res)
 {
   if (!engine_name)
   {

sql/mysqld.h

@@ -146,8 +146,8 @@ extern ulong opt_replicate_events_marked_for_skip;
 extern char *default_tz_name;
 extern Time_zone *default_tz;
 extern char *my_bind_addr_str;
-extern char *default_storage_engine, *default_tmp_storage_engine;
-extern char *enforced_storage_engine;
+extern const char *default_storage_engine, *default_tmp_storage_engine;
+extern const char *enforced_storage_engine;
 extern char *gtid_pos_auto_engines;
 extern plugin_ref *opt_gtid_pos_auto_plugins;
 extern bool opt_endinfo, using_udf_functions;

sql/sys_vars.inl

@@ -1640,7 +1640,7 @@ class Sys_var_plugin: public sys_var
   Sys_var_plugin(const char *name_arg,
           const char *comment, int flag_args, ptrdiff_t off, size_t size,
           CMD_LINE getopt,
-          int plugin_type_arg, char **def_val, PolyLock *lock=0,
+          int plugin_type_arg, const char **def_val, PolyLock *lock=0,
           enum binlog_status_enum binlog_status_arg=VARIABLE_NOT_IN_BINLOG,
           on_check_function on_check_func=0,
           on_update_function on_update_func=0,