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/ctx.hpp

@@ -187,6 +187,9 @@ namespace zmq
         //  Number of I/O threads to launch.
         int io_thread_count;
 
+        //  Does context wait (possibly forever) on termination?
+        bool blocky;
+
         //  Is IPv6 enabled on this context?
         bool ipv6;