Fix LIBZMQ-404: zmq_term not truly re-entrant

zmq_term can not safely be re-entered with pgm transport.
Fix proposed by Steven McCoy.

src/zmq.cpp

@@ -173,6 +173,8 @@ int zmq_ctx_destroy (void *ctx_)
     int rc = ((zmq::ctx_t*) ctx_)->terminate ();
     int en = errno;
 
+    //  Shut down only if termination was not interrupted by a signal.
+    if (!rc || en != EINTR) {
 #ifdef ZMQ_HAVE_WINDOWS
         //  On Windows, uninitialise socket layer.
         rc = WSACleanup ();
@@ -184,6 +186,7 @@ int zmq_ctx_destroy (void *ctx_)
         if (pgm_shutdown () != TRUE)
             zmq_assert (false);
 #endif
+    }
 
     errno = en;
     return rc;