remove return value annotation from unbase

unbase is used for :8(...) and the like, but it can also return a Failure
when the argument is not in a numeric format. The Numeric:D return type
then makes it throw an exception, hiding the actual problem
Avoid EVAL in IO.umask

For interpreting a string as an octal number, :8(...) should be much faster,
and it is not prone to injection attacks.