Add I/O manager API notifyIOManagerCapabilitiesChanged

Used in setNumCapabilities.

It only does anything for MIO on Posix.

Previously it always invoked Haskell code, but that code only did
anything on non-Windows (and non-JS), and only threaded. That currently
effectively means the MIO I/O manager on Posix.

So now it only invokes it for the MIO Posix case.