As of commit 90dcbbe (PHP-7.2+) bcmod() supports non-integral
parameters as well. Since formerly only integer modulus has been
supported, it did not make much sense to cater to the scale with regard
to the result. However, now it does for consistency with other BCMath
operations.
Therefore, we add support for an optional `scale` parameter and fall
back to the default scale (`bcmath.scale`) as usual.
bcmod() no longer truncates fractionals to integers. This matches
the behavior of fmod(). It also matches the behavior of bcpowmod().
It also matches the behavior of bcmod() in HHVM.
