s390 && user_enable_single_step() (Was: odd utrace testing results on s390x)

[Roland McGrath]

> Ok, I changed the wording slightly:
> 
> Clear the TIF_SINGLE_STEP bit in copy_thread. The new process did not get
> a PER event of its own. It is wrong deliver a SIGTRAP that was meant for
> the parent process.

Very good!

Thanks,
Roland