Problem: cast to unsigned limited result to 
max signed bigint 9223372036854775808,
instead of max unsigned bigint 18446744073709551615.

Fix: don't use args[0]->val_int() when casting from
a floating point number, use val() instead, with range checkings,
special to unsigned data type.

item_func.cc:
  Special handling of cast from REAL_RESULT
  to unsigned int: we cannot execute args[0]->val_int()
  because it cuts max allowed value to LONGLONG_INT,
  instead of ULONGLONG_INT required.
count_distinct3.test:
  Getting rid of "Data truncated; out of range ..." warnings.
cast.test, cast.result:
  Adding test case.
ps.result:
  Fixing that cast from 6570515219.6535 
  to unsigned didn't round to 6570515220,
  and returned 6570515219 instead.