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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/udhcp/dhcpc.c

@@ -140,7 +140,7 @@ static uint8_t* alloc_dhcp_option(int code, const char *str, int extra)
 }
 
 
-int udhcpc_main(int argc, char **argv);
+int udhcpc_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int udhcpc_main(int argc, char **argv)
 {
 	uint8_t *temp, *message;

networking/udhcp/dhcpd.c

@@ -21,7 +21,7 @@ struct dhcpOfferedAddr *leases;
 /* struct server_config_t server_config is in bb_common_bufsiz1 */
 
 
-int udhcpd_main(int argc, char **argv);
+int udhcpd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int udhcpd_main(int argc, char **argv)
 {
 	fd_set rfds;

networking/udhcp/dhcprelay.c

@@ -279,7 +279,7 @@ static void dhcprelay_loop(int *fds, int num_sockets, int max_socket, char **cli
 	}
 }
 
-int dhcprelay_main(int argc, char **argv);
+int dhcprelay_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dhcprelay_main(int argc, char **argv)
 {
 	int num_sockets, max_socket;

networking/udhcp/dumpleases.c

@@ -7,7 +7,7 @@
 #include "common.h"
 #include "dhcpd.h"
 
-int dumpleases_main(int argc, char **argv);
+int dumpleases_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dumpleases_main(int argc, char **argv)
 {
 	int fd;