Problem: linger values other than -1 or 0 are unsafe

Solution: set defaults back to infinity, and add new context
option, ZMQ_BLOCKY that the user can set to false to get a
less surprising behavior on context termination. Eg.

    zmq_ctx_set (ctx, ZMQ_BLOCKY, false);

src/options.cpp

@@ -35,7 +35,7 @@ zmq::options_t::options_t () :
     rcvbuf (0),
     tos (0),
     type (-1),
-    linger (30000),
+    linger (-1),
     reconnect_ivl (100),
     reconnect_ivl_max (0),
     backlog (100),