ZMQ_LINGER socket option added.

1. ZMQ_LINGER option can be set/get
    2. options are part of own_t base class rather than being declared
       separately by individual objects
    3. Linger option is propagated with "term" command so that the
       newest value of it is used rather than the stored old one.
    4. Session sets the linger timer if needed and terminates
       as soon as it expires.
    5. Corresponding documentation updated.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/pub.hpp

@@ -47,7 +47,7 @@ namespace zmq
     private:
 
         //  Hook into the termination process.
-        void process_term ();
+        void process_term (int linger_);
 
         //  Write the message to the pipe. Make the pipe inactive if writing
         //  fails. In such a case false is returned.