Silence all "unused parameter" warnings from Clang.

Compiling without warnings is a good goal, because it makes
new warnings (which probably indicate bugs) stand out rather
than getting lost in the spam.

My fixes fall into two categories:

    - Adding (void) casts of unused parameters, where their
      unusedness seems like a TODO (or in some cases a bug?).

    - Removing parameter names altogether, where the function
      is clearly a stub that will never use its parameters.

Should be no change in behavior.

src/router.cpp

@@ -61,6 +61,9 @@ zmq::router_t::~router_t ()
 
 void zmq::router_t::xattach_pipe (pipe_t *pipe_, bool icanhasall_)
 {
+    // icanhasall_ is unused
+    (void)icanhasall_;
+
     zmq_assert (pipe_);
 
     bool identity_ok = identify_peer (pipe_);
@@ -128,6 +131,9 @@ void zmq::router_t::xwrite_activated (pipe_t *pipe_)
 
 int zmq::router_t::xsend (msg_t *msg_, int flags_)
 {
+    // flags_ is unused
+    (void)flags_;
+
     //  If this is the first part of the message it's the ID of the
     //  peer to send the message to.
     if (!more_out) {
@@ -193,6 +199,9 @@ int zmq::router_t::xsend (msg_t *msg_, int flags_)
 
 int zmq::router_t::xrecv (msg_t *msg_, int flags_)
 {
+    // flags_ is unused
+    (void)flags_;
+
     if (prefetched) {
         if (!identity_sent) {
             int rc = msg_->move (prefetched_id);