A couple bug-fixes.

Truncate single-precision NaNs and don't flush inputs to zero. Tested
against hardware.
Fix FPRF for floats.  Fix a couple bugs with very small numbers.
Switch FPRF updates to new, clearer API.
Fix flags for denormals in non-IEEE mode.
Fix obvious typo in round-to-infinite code.
Refactor rounding to be more intuitive.

Also fixes flags for flushed denormals.