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/xpub.cpp

@@ -131,6 +131,9 @@ bool zmq::xpub_t::xhas_out ()
 
 int zmq::xpub_t::xrecv (msg_t *msg_, int flags_)
 {
+    // flags_ is unused
+    (void)flags_;
+
     //  If there is at least one 
     if (pending.empty ()) {
         errno = EAGAIN;