Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC various fma{,f,l} improvements:

r226245 - refactoring
r226371 - fix double-rounding bug
r226373 - new math_private.h macros
r226601 - fix nit in r226371

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Fix some corner cases:
- fma(x, y, z) returns z, not NaN, if z is infinite, x and y are finite,
x*y overflows, and x*y and z have opposite signs.
- fma(x, y, z) doesn't generate an overflow, underflow, or inexact exception
if z is NaN or infinite, as per IEEE 754R.
- If the rounding mode is set to FE_DOWNWARD, fma(1.0, 0.0, -0.0) is -0.0,
not +0.0.

Implement modfl().

Fix the double rounding problem with subnormals, and
remove the XXX comments, which no longer apply.

Implement fmal.