#4853 floattofp truncates instead of rounding, and asymmetrically

[David Scherfgen]

Spotted by Claude:

(%i1) fpprec: 15$

(%i2) bfloat(2.0/3.0) + bfloat(-2.0/3.0);
(%o2) -2.22044604925031b-16

Should be 0b0.