relayd: track the health quit pipe with the fd-tracker

Create the relayd health thread quit pipe through the
fd_tracker_util_pipe_open_cloexec() function which wraps
utils_create_pipe_cloexec(), but tracks the resulting file descriptor.

The close of the pipe is also performed through the fd tracker.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib82eabc0f152a706ac76a5042af2e3984d7e7f72