Style: Fix unused parameter compilation warnings

This commit addresses the following warnings reported on gcc 5.2.1. In
the future, this will help reduce the "noise" and help catch warnings
revealing a serious problem.

8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---
/path/to/libzmq/src/options.cpp:1048:36: warning: unused parameter ‘option_’ [-Wunused-parameter]
 bool zmq::options_t::is_valid (int option_) const
                                    ^

/path/to/libzmq/src/plain_client.cpp:146:30: warning: unused parameter ‘cmd_data’ [-Wunused-parameter]
         const unsigned char *cmd_data, size_t data_size)
                              ^

/path/to/libzmq/src/plain_client.cpp:146:30: warning: unused parameter ‘cmd_data’ [-Wunused-parameter]
         const unsigned char *cmd_data, size_t data_size)
                              ^

/path/to/libzmq/src/socket_base.cpp:1445:44: warning: unused parameter ‘group_’ [-Wunused-parameter]
 int zmq::socket_base_t::xjoin (const char *group_)
                                            ^
/path/to/libzmq/src/socket_base.cpp:1451:45: warning: unused parameter ‘group_’ [-Wunused-parameter]
 int zmq::socket_base_t::xleave (const char *group_)
                                             ^

/path/to/libzmq/src/radio.cpp:145:33: warning: unused parameter ‘msg_’ [-Wunused-parameter]
 int zmq::radio_t::xrecv (msg_t *msg_)
                                 ^
/path/to/libzmq/src/dish.cpp:164:32: warning: unused parameter ‘msg_’ [-Wunused-parameter]
 int zmq::dish_t::xsend (msg_t *msg_)
                                ^

/path/to/libzmq/tests/test_msg_ffn.cpp:32:16: warning: unused parameter ‘data’ [-Wunused-parameter]
 void ffn(void *data, void *hint) {
                ^

/path/to/libzmq/tests/test_timers.cpp:50:19: warning: unused parameter ‘timer_id’ [-Wunused-parameter]
 void handler (int timer_id, void* arg)
                   ^
8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---

src/dish.cpp

@@ -163,6 +163,7 @@ int zmq::dish_t::xleave (const char* group_)
 
 int zmq::dish_t::xsend (msg_t *msg_)
 {
+    LIBZMQ_UNUSED (msg_);
     errno = ENOTSUP;
     return -1;
 }

src/options.cpp

@@ -31,6 +31,7 @@
 
 #include "options.hpp"
 #include "err.hpp"
+#include "macros.hpp"
 #include "../include/zmq_utils.h"
 
 zmq::options_t::options_t () :
@@ -1047,5 +1048,6 @@ int zmq::options_t::getsockopt (int option_, void *optval_, size_t *optvallen_)
 
 bool zmq::options_t::is_valid (int option_) const
 {
+    LIBZMQ_UNUSED (option_);
     return true;
 }

src/plain_client.cpp

@@ -27,6 +27,7 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include "macros.hpp"
 #include "platform.hpp"
 #ifdef ZMQ_HAVE_WINDOWS
 #include "windows.hpp"
@@ -145,6 +146,8 @@ int zmq::plain_client_t::produce_hello (msg_t *msg_) const
 int zmq::plain_client_t::process_welcome (
         const unsigned char *cmd_data, size_t data_size)
 {
+    LIBZMQ_UNUSED (cmd_data);
+
     if (state != waiting_for_welcome) {
         errno = EPROTO;
         return -1;

src/radio.cpp

@@ -145,6 +145,7 @@ bool zmq::radio_t::xhas_out ()
 int zmq::radio_t::xrecv (msg_t *msg_)
 {
     //  Messages cannot be received from PUB socket.
+    LIBZMQ_UNUSED (msg_);
     errno = ENOTSUP;
     return -1;
 }

src/socket_base.cpp

@@ -1444,12 +1444,14 @@ bool zmq::socket_base_t::xhas_in ()
 
 int zmq::socket_base_t::xjoin (const char *group_)
 {
+    LIBZMQ_UNUSED (group_);
     errno = ENOTSUP;
     return -1;
 }
 
 int zmq::socket_base_t::xleave (const char *group_)
 {
+    LIBZMQ_UNUSED (group_);
     errno = ENOTSUP;
     return -1;
 }

tests/test_msg_ffn.cpp

@@ -27,9 +27,11 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include "macros.hpp"
 #include "testutil.hpp"
 
 void ffn(void *data, void *hint) {
+    LIBZMQ_UNUSED (data);
     // Signal that ffn has been called by writing "freed" to hint
     memcpy(hint, (void *) "freed", 5);
 }

tests/test_timers.cpp

@@ -27,6 +27,7 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include "macros.hpp"
 #include "testutil.hpp"
 #include "../include/zmq_utils.h"
 
@@ -49,6 +50,7 @@ void sleep_ (long timeout_)
 
 void handler (int timer_id, void* arg)
 {
+    LIBZMQ_UNUSED (timer_id);
     *((bool *)arg) = true;
 }