For older kernels (e.g. RHEL7 with 3.10) it seems that wait(NULL) after
ptrace(PTHREAD_ATTACH) does not work properly for threads that have
to be created via clone().

Fix this by using waitpid() with the __WALL flag.

>From the waitpid() man page:

 __WALL (since Linux 2.4)
 Wait  for  all  children, regardless of type ("clone" or "non-clone").
Reported-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>