add threadsafe version of inet_ntoa (inet_ntoa_r takes a buffer to fill)

this is used by some debugging functions

sys/libkern/inet_ntoa.c

@@ -48,3 +48,17 @@ inet_ntoa(struct in_addr ina)
 	return buf;
 }
 
+char *
+inet_ntoa_r(struct in_addr ina, char *buf)
+{
+	unsigned char *ucp = (unsigned char *)&ina;
+
+	sprintf(buf, "%d.%d.%d.%d",
+		ucp[0] & 0xff,
+		ucp[1] & 0xff,
+		ucp[2] & 0xff,
+		ucp[3] & 0xff);
+	return buf;
+}
+
+