We change `bcmul()` and `bcpow()` so that the result has exactly the
requested scale (i.e. decimal places) to make them consistent with the
other BCMath functions. This also changes our stance regarding bug
#52748, which had been classified as documentation problem.
We do not manipulate the numbers themselves (anymore), but rather
introduce `bc_num2str_ex()` which accepts a scale parameter that
overrides the scale of the number by omitting extraneous decimals and
adding zeros, respectively. This also allows us to get rid of
`split_bc_num()`, which fixes bug #75164 as well.
