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/tcp.cpp

@@ -63,6 +63,15 @@ void zmq::tune_tcp_socket (fd_t s_)
 
 void zmq::tune_tcp_keepalives (fd_t s_, int keepalive_, int keepalive_cnt_, int keepalive_idle_, int keepalive_intvl_)
 {
+    // These options are used only under certain #ifdefs below.
+    (void)keepalive_;
+    (void)keepalive_cnt_;
+    (void)keepalive_idle_;
+    (void)keepalive_intvl_;
+
+    // If none of the #ifdefs apply, then s_ is unused.
+    (void)s_;
+
     //  Tuning TCP keep-alives if platform allows it
     //  All values = -1 means skip and leave it for OS
 #ifdef ZMQ_HAVE_SO_KEEPALIVE