r82614: add functionality to change FPU rounding mode (via float.__setround__ and float.__getround__ functions), on platforms that support the standard C99 fesetround and fegetround functions:

>>> float.__getround__()
'tonearest'
>>> 1e300 * 1e300
inf
>>> float.__setround__("downward")
>>> 1e300 * 1e300
1.7976931348623157e+308

This is just temporary, so that I can test that FPU rounding mode doesn't affect results of string-to-float and float-to-string conversions.  I'm not planning to merge any of r82614 back to py3k.