This patch makes the following changes (according to the task description):
- Adds Type_handler::Item_func_round_fix_length_and_dec().
- Splits the code from Item_func_round::fix_length_and_dec() into new
  Item_func_round methods fix_arg_int(), fix_arg_decimal(), fix_arg_double().
- Calls the new Item_func_round methods from the relevant implementations of
  Type_handler_xxx::Item_func_round_fix_length_and_dec().
- Adds a new error message ER_ILLEGAL_PARAMETER_DATA_TYPE_FOR_OPERATION
- Makes ROUND() return the new error for GEOMETRY

Additionally:
- Inherits Item_func_round directly from Item_func_numhybrid as it
  uses nothing from Item_func_num1.
- Fixes "MDEV-12000 ROUND(expr,const_expr_returning_NULL) creates DOUBLE(0,0)".
  Now if args[1] returns NULL, the data type is set to DOUBLE with
  NOT_FIXED_DEC decimals instead of 0 decimals.