enable -Wpointer-arith, -Wstrict-prototypes

Also enable -Wunused-parameter - this forces us to add the gcc
"unused" attribute to a lot of parameters (mostly library callback
functions), but it's worth it during code refactorizations.

src/sig_handlers.c

@@ -47,7 +47,7 @@ int handlePendingSignals(void)
 	return 0;
 }
 
-static void chldSigHandler(int sig)
+static void chldSigHandler(mpd_unused int sig)
 {
 	int status;
 	int pid;