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/zmq_init.hpp

@@ -25,7 +25,6 @@
 #include "own.hpp"
 #include "fd.hpp"
 #include "stdint.hpp"
-#include "options.hpp"
 #include "stdint.hpp"
 #include "blob.hpp"
 
@@ -76,9 +75,6 @@ namespace zmq
         //  Identity of the peer socket.
         blob_t peer_identity;
 
-        //  Associated socket options.
-        options_t options;
-
         //  I/O thread the object is living in. It will be used to plug
         //  the engine into the same I/O thread.
         class io_thread_t *io_thread;