nvidia 304 and fc23: black screens after updating today

[John Pilkington]

This is the same system that had nvidia problems during its fc22 > fc23 
upgrade.  That was reported here and solved on 30 June.  Since then it 
has been working well, but after installing the new packages today I get 
black screens with kernels 4.7.3, 4.7.5 and 4.7.6

'dnf downgrade xorg-x11-drv-nvidia-304xx --allowerasing' apparently 
completed but then akmods-shutdown errored.  After re-upgrading, 
akmods-shutdown completed, and the nvidia flash screen flashes, but then 
all goes black again, for all installed kernels.  I'm using tty2.

Xorg.0.log looks ok apart from 'ACPI: failed to connect to the ACPI 
event daemon' and suggests looking at the ConnectToAcpid and 
AcpidSocketPath options, which I have not yet done.  I don't know if 
this is likely to be the real problem.

John P