Replace `masking exceptions' by `trapping exceptions'.

Replace `masking exceptions' by `trapping exceptions'.

Reversing the sense makes it clearer -- `trap on division by zero' is
a positive request, versus `mask division by zero traps', i.e. `don't
trap on division by zero'.  This better reflects the IEEE 754 notion
of actions to take on exceptions.  Also, the sense of `mask' is often
confusing (Scheme's `interrupt mask' is not the interrupts that are
masked, but the bit mask of interrupts that are *enabled*; the BSD
fpgetmask and fpsetmask library routines control the bit mask of
floating-point exceptions for which trapping is enabled; but, on the
other hand, a POSIX process's `signal mask' is the set of signals
that are disabled).