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.h

@@ -4832,7 +4832,7 @@ public:
   CHARSET_INFO *cs;
   sql_exchange(const char *name, bool dumpfile_flag,
                enum_filetype filetype_arg= FILETYPE_CSV);
-  bool escaped_given(void);
+  bool escaped_given(void) const;
 };
 
 /*