Leave our wakeup signal blocked after destroying the ShmEventLoop

This will avoid late-arriving wakeup signals killing the process
(instead of letting it shut down normally).

Change-Id: Ie87c3535c6cb884df270f268fe8b02ea9118a245
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/ipc_lib/signalfd.h b/aos/ipc_lib/signalfd.h
index a2c2549..3afdf99 100644
--- a/aos/ipc_lib/signalfd.h
+++ b/aos/ipc_lib/signalfd.h
@@ -26,6 +26,11 @@
   // will be 0.
   signalfd_siginfo Read();
 
+  // Ensures the destructor will leave the specific signal blocked. This can be
+  // helpful if the signal is sent asynchronously, such that it may arrive after
+  // this object is destroyed, to ensure that doesn't kill the process.
+  void LeaveSignalBlocked(unsigned int signal);
+
  private:
   int fd_ = -1;