decimal->ulong conversion fixed to assign max possible ULONG if decimal
is bigger
Item_func_unsigned now handles DECIMAL parameter separately as we can't
rely on decimal::val_int result here.


mysql-test/r/type_newdecimal.result:
  result fixed
mysql-test/t/type_newdecimal.test:
  testcase
sql/item_func.cc:
  DECIMAL_RESULT should be handled separately here as it's
  always signed.
strings/decimal.c:
  here we assign max possible ULONG if the decimal value is bigger