This is required due to lack of a graceful shutdown: there is a small gap
between the runtime's memory pool release and router process's exit. Thus, a
worker thread may start processing a request between these two operations,
which may result in an http fields hash access and subsequent crash.

To simplify issue reproduction, it makes sense to add a 2 sec sleep before
exit() in nxt_runtime_exit().

After 41331471eee7 completion handlers should complete next buffer in chain.
Otherwise buffer memory may leak.

Thanks to Peter Tkatchenko for reporing the issue and testing fixes.

An earlier attempt (ad6265786871) to resolve this condition on the
router's side added a new issue: the app could get a request before
acquiring a port.

One of the ways to detect Unit's startup and subsequent readiness to accept
commands relies on waiting for the control socket file to be created.
Earlier, it was unreliable due to a race condition between the client's
connect() and the daemon's listen() calls after the socket's bind() call.

Now, unix domain listening sockets are created with a nxt_listen_socket_create()
call as follows:

   s = socket();
   unlink("path/to/socket.tmp")
   bind(s, "path/to/socket.tmp");
   listen(s);
   rename("path/to/socket.tmp", "path/to/socket");

This eliminates a time-lapse when the socket file is already created but nobody
is listening on it yet, which therefore prevents the condition described above.

Also, it allows reliably detecting whether the socket is being used or simply
wasn't cleaned after the daemon stopped abruptly.  A successful connection to
the socket file means the daemon has been started; otherwise, the file can be
overwritten.

Previously, the unix domain control socket file might have been left
in the file system after a failed nxt_listen_socket_create() call.