LIBZMQ-195 allow explicitly setting sndbuf and rcvbuf to 0 (see https://support.microsoft.com/en-us/kb/201213)

src/options.cpp

@@ -41,8 +41,8 @@ zmq::options_t::options_t () :
     rate (100),
     recovery_ivl (10000),
     multicast_hops (1),
-    sndbuf (0),
-    rcvbuf (0),
+    sndbuf (-1),
+    rcvbuf (-1),
     tos (0),
     type (-1),
     linger (-1),