Add separate type handlers for unsigned integer data types

Let's split numeric type handlers into separate signed and unsigned variants. This is needed:

• for better error messages, e.g. bigint unsigned vs just bigint in this example:

  MariaDB [test]> select @@max_allowed_packet+ROW(1,1);

  ERROR 4078 (HY000): Illegal parameter data types bigint unsigned and row for operation '+'

• for function collection plugins, to define function prototypes easier.
• for data type conversion plugins, a signed and unsigned int types have a different internal representation

Let's add new classes handling unsigned data types (deriving from their signed counterparts):

• Type_handler_utiny
• Type_handler_ushort
• Type_handler_uint24
• Type_handler_ulong
• Type_handler_ulonglong

Add new global variables:

• type_handler_utiny
• type_handler_ushort
• type_handler_uint24
• type_handler_ulong
• type_handler_ulonglong

Rename the old variables to:

• type_handler_stiny
• type_handler_sshort
• type_handler_sint24
• type_handler_slong
• type_handler_slonglong
  (i.e. add the 's' prefix on the type name)