Changes:

	- adapt to "no more utrace changes". Whatever I think
	  about utrace I have to agree, it is not practical to
	  change utrace now

	- attach to the thread-group with the dead leader

	- fix the double-attaching bug

	- do not assume we can trust pid_task(), the tracee can
	  be reaped even if it can't pass ->report_death()

	- size_t is unsigned! -EAGAIN from ->read() makes this
	  fd unusable from rw_verify_area() pov

Next: report signals.