dev-disconnect: add blackhole option.

To reproduce the next bug, I had to ensure that one node keeps thinking it's
disconnected, then the other node reconnects, then the first node realizes
it's disconnected.

This code does that, adding a '0' dev-disconnect modifier.  That means
we fork off a process which (due to pipebuf) will accept a little
data, but when the dev_disconnect file is truncated (a hacky, but
effective, signalling mechanism) will exit, as if the socket finally
realized it's not connected any more.

The python tests hang waiting for the daemon to terminate if you leave
the blackhole around; to give a clue as to what's happening in this
case I moved the log dump to before killing the daemon.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/utils.py

@@ -59,15 +59,15 @@ class TailableProc(object):
         self.running = True
 
     def stop(self):
-        self.proc.terminate()
-        self.proc.kill()
-        self.proc.wait()
-        self.thread.join()
         if self.outputDir:
             logpath = os.path.join(self.outputDir, 'log')
             with open(logpath, 'w') as f:
                 for l in self.logs:
                     f.write(l + '\n')
+        self.proc.terminate()
+        self.proc.kill()
+        self.proc.wait()
+        self.thread.join()
 
     def tail(self):
         """Tail the stdout of the process and remember it.