Remove OA_DANGEROUS from exit

Remove OA_DANGEROUS from exit

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

exit usually doesn’t return.  When it fails, it returns a read-only
undef, so we don’t need temp copies for its sake.