Fixed 'make check' failures

- Split off NULL security check from PLAIN
- Cleaned up test_linger code a little
- Got all tests to pass, added TODOs for outstanding issues
- Added ZAP authentication for NULL test case
- NULL mechanism was not passing server identity - fixed
- cleaned up test_security_plain and removed option double-checks (made code ugly)
- lowered timeout on expect_bounce_fail to 150 msec to speed up checks
- removed all sleeps from test_fork and simplified code (it still passes :-)

src/null_mechanism.cpp

@@ -196,6 +196,14 @@ void zmq::null_mechanism_t::send_zap_request ()
     rc = session->write_zap_msg (&msg);
     errno_assert (rc == 0);
 
+    //  Identity frame 
+    rc = msg.init_size (options.identity_size);
+    errno_assert (rc == 0);
+    memcpy (msg.data (), options.identity, options.identity_size);
+    msg.set_flags (msg_t::more);
+    rc = session->write_zap_msg (&msg);
+    errno_assert (rc == 0);
+
     //  Mechanism frame
     rc = msg.init_size (5);
     errno_assert (rc == 0);