Adding "const" qualifier into a few methods and parameters in the LOAD code

Methods:
- Item_user_var_as_out_param::print_for_load()
- sql_exchange::escaped_given(void)

Parameters:
- sql_exchange in write_execute_load_query_log_event()
- sql_exchange in mysql_load()
- sql_exchange in Load_log_event::Load_log_event()

Also, removing cast to "char*" in a few places in
Load_log_event::Load_log_event()

sql/sql_class.cc

@@ -2945,7 +2945,7 @@ sql_exchange::sql_exchange(const char *name, bool flag,
   cs= NULL;
 }
 
-bool sql_exchange::escaped_given(void)
+bool sql_exchange::escaped_given(void) const
 {
   return escaped != &default_escaped;
 }