fix(console): a serial console worker holds only its own terminal

pty.fork returns an inheritable master descriptor, so each new console
worker, and the ssh it may start, held the terminals of every other open
session until it ended. A worker now closes every descriptor above its own
terminal before it starts.

console/socketiod

@@ -52,6 +52,9 @@ def connect(sid, environ, auth):
     pid, fd = pty.fork()
     if pid == 0:
         try:
+            # pty.fork's master is inheritable: without this each worker would
+            # hold the other sessions' consoles open
+            os.closerange(3, os.sysconf("SC_OPEN_MAX"))
             os.execv(sys.executable, [sys.executable, CONSOLECALLBACK, str(instance.pk)])
         finally:
             os._exit(1)