BUG/MINOR: threads: always set an owner to the thread_sync pipe

BUG/MINOR: threads: always set an owner to the thread_sync pipe

The owner of the fd used by the synchronization pipe was set to NULL,
making it ignored by maxfd computation. The risk would be that some
synchronization events get delayed between threads when using poll()
or select(). However this is only theorical since the pipe is created
before listeners are bound so normally its FD should be lower and
this should normally not happen. The only possible situation would
be if all listeners are bound to inherited FDs which are lower than
the pipe's.

This patch must be backported to 1.8.