Add "draft" capability to zmq_has

As discussed in #2307

src/zmq.cpp

@@ -1516,6 +1516,10 @@ int zmq_has (const char *capability)
 #if defined (ZMQ_HAVE_VMCI)
     if (strcmp (capability, "vmci") == 0)
         return true;
+#endif
+#if defined (ZMQ_BUILD_DRAFT_API)
+    if (strcmp (capability, "draft") == 0)
+        return true;
 #endif
     //  Whatever the application asked for, we don't have
     return false;