relayd: track the quit pipe with the fd-tracker

Create the relayd 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: Ia2a000030765e8df990e0ea3d35a8bf157630d9f