add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/dnsd.c

@@ -328,7 +328,7 @@ static void interrupt(int x)
 	exit(2);
 }
 
-int dnsd_main(int argc, char **argv);
+int dnsd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dnsd_main(int argc, char **argv)
 {
 	const char *listen_interface = "0.0.0.0";