[Development] Backporting the Keccak change

[Thiago Macieira]

When writing the 5.6.3 changelog, I'm currently leaving it with:

******************************************************************************
*                     Important Behavior Changes                             *
******************************************************************************

 - QCryptographicHash:
   * [QTBUG-59770] QCryptographicHash now properly calculates SHA3 message
     digests. Before, when asked to calculate a SHA3 digest, it calculated
     a Keccak digest instead.

I think this is bad for the 5.6.3 release. After 5.9.0 was released, we had a 
couple of people asking for the ability to calculate Keccak instead of SHA3 
because they needed to compare against stored hashes that had been 
(incorrectly) calculated using Keccak.

So I think we need to take action here. But what?

 a) revert the 5.6 backport of 88a8feeacb9bdaff9ee06164424e407eb904cd10 so 
that 5.6.x will forever calculate Keccak, not SHA3;

 b) additionally backport 12c5264d9add1826d543c36d893db77262195fc6 to both 5.6 
and 5.9, with the proper binary compatibility notices, so that people who need 
to can adapt their code to calculate Keccak. It won't be pretty, but it will 
work.

I'm actually leaning towards (a) for 5.6 and (b) for 5.9.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center