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/curve_server.cpp

@@ -537,7 +537,7 @@ void zmq::curve_server_t::send_zap_request (const uint8_t *key)
     rc = session->write_zap_msg (&msg);
     errno_assert (rc == 0);
 
-    // identity frame 
+    //  Identity frame 
     rc = msg.init_size (options.identity_size);
     errno_assert(rc == 0);
     memcpy (msg.data (), options.identity, options.identity_size);