Fix floating point handling: When converting double to size_t,

Fix floating point handling: When converting double to size_t,
properly round to the nearest M (=0.001m), which is the smallest
available unit.

This avoids weirdness like (size_t)(0.6 * 10.0) == 5
by instead calculating (size_t)(0.6 * 10.0 + 0.0005) == 6,
and so it fixes the indentation of the readline(3) manual.