Controller: improved handling of unix domain control socket.

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.

test/unit/main.py

@@ -185,10 +185,7 @@ class TestUnit(unittest.TestCase):
 
         atexit.register(self.stop)
 
-        # Due to race between connect() and listen() after the socket binding
-        # tests waits for unit.pid file which is created after listen().
-
-        if not self.waitforfiles(self.testdir + '/unit.pid'):
+        if not self.waitforfiles(self.testdir + '/control.unit.sock'):
             exit("Could not start unit")
 
         self.skip_alerts = [