Well, that was an unedifying 'update experience'... I have committed the unpardonable sin of having my home directory on another disk than the system disk. Consequently, the secinitd process seemed to have had a little spasm of trying to repair library permissions after the update, giving rise to a password prompt for
each of the affected libraries (hundreds of them). After more than an hour of typing in my password in these prompts (it could not be pasted in), I got a little impatient and tried to restart. Then my Mac wouldn't boot (thanks Apple) and went into recovery mode. I used disk utility to repair permissions on my system disk, which detected several group permission errors, and then tried again to boot. No dice. I had to restore the system from my Time Machine backup, but thankfully it had a backup point for 10.10.5, so I tired that. Now everything works. I guess repairing permissions must have solved the problem so that 10.10.5 works, but I fail to understand how the error arose in the first place. I have never changed group permissions.
WTF
???