mailbox::recv correctly passes EINTR to the caller

This bug caused signal during blocking call to assert.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/mailbox.cpp

@@ -66,7 +66,7 @@ int zmq::mailbox_t::recv (command_t *cmd_, int timeout_)
 
     //  Wait for signal from the command sender.
     int rc = signaler.wait (timeout_);
-    if (rc != 0 && errno == EAGAIN)
+    if (rc != 0 && (errno == EAGAIN || errno == EINTR))
         return -1;
 
     //  We've got the signal. Now we can switch into active state.