Minor fixes (#618)

* Prevent double conversion

The size would be converted from int to size_t and then back to int
when calling the function backtrace_symbols_fd().

* Remove deprecated setsockop

src/xutils.cpp

@@ -113,7 +113,7 @@ namespace xpyt
         void* array[10];
 
         // get void*'s for all entries on the stack
-        size_t size = backtrace(array, 10);
+        int size = backtrace(array, 10);
 
         // print out all the frames to stderr
         fprintf(stderr, "Error: signal %d:\n", sig);

test/xeus_client.cpp

@@ -95,12 +95,12 @@ nl::json xeus_client_base::receive_on_control()
 
 void xeus_client_base::subscribe_iopub(const std::string& filter)
 {
-    m_iopub.setsockopt(ZMQ_SUBSCRIBE, filter.c_str(), filter.length());
+    m_iopub.set(zmq::sockopt::subscribe, filter);
 }
 
 void xeus_client_base::unsubscribe_iopub(const std::string& filter)
 {
-    m_iopub.setsockopt(ZMQ_UNSUBSCRIBE, filter.c_str(), filter.length());
+    m_iopub.set(zmq::sockopt::unsubscribe, filter);
 }
 
 nl::json xeus_client_base::receive_on_iopub()