zebra: deal with irdp compile warnings

* zebra/irdp.h
  * irdp_send_thread(): move prototype to common header file
  * irdp_advert_off(): idem
  * process_solicit(): idem
  * irdp_read_raw(): idem
  * send_packet(): idem
* zebra/irdp_interface.c
  * inet_2a(): move function to where it is used
  * in_cksum(): lib/checksum.h already provides prototype
  * irdp_send_thread(): prototype moved away
  * inet_2a(): idem
  * irdp_advert_off(): idem
  * b1, b2, b3, b4: get rid of global buffers for inet_2a()
  * if_group(): add local buffer for inet_2a()
  * if_add_group(): idem
  * if_drop_group(): idem
  * irdp_config_write(): idem
  * Adv_new(): make static
* zebra/irdp_main.c
  * irdp_read_raw(): prototype moved away
  * send_packet(): idem
  * in_cksum(): lib/checksum.h already provides prototype
  * inet_2a(): function moved to irdp_interface.c
* zebra/irdp_packet.c
  * b1, b2, b3, b4: get rid of global buffers for inet_2a()
  * in_cksum(): lib/checksum.h already provides prototype
  * process_solicit(): prototype moved away
  * irdp_read_raw(): fix uninitialized variable
diff --git a/zebra/irdp_main.c b/zebra/irdp_main.c
index 5510669..c297979 100644
--- a/zebra/irdp_main.c
+++ b/zebra/irdp_main.c
@@ -60,6 +60,7 @@
 #include "zebra/irdp.h"
 #include <netinet/ip_icmp.h>
 
+#include "checksum.h"
 #include "if.h"
 #include "sockunion.h"
 #include "log.h"
@@ -75,25 +76,6 @@
 /* Timer interval of irdp. */
 int irdp_timer_interval = IRDP_DEFAULT_INTERVAL;
 
-int irdp_read_raw(struct thread *r);
-int in_cksum (void *ptr, int nbytes);
-void send_packet(struct interface *ifp, 
-		 struct stream *s,
-		 u_int32_t dst,
-		 struct prefix *p,
-		 u_int32_t ttl);
-
-char *
-inet_2a(u_int32_t a, char *b)
-{
-  sprintf(b, "%u.%u.%u.%u", 
-          (a    ) & 0xFF,
-          (a>> 8) & 0xFF, 
-          (a>>16) & 0xFF, 
-          (a>>24) & 0xFF);
-  return  b;
-}
-
 int
 irdp_sock_init (void)
 {