Implemented initial UDP socket.

net/p2pnet.h

@@ -13,4 +13,6 @@
    int socket_tcp4(void);
 
    int socket_stream_sctp4(void);
+
+   int socket_udp4(void);
 #endif // P2PNET_H

net/udp.c

@@ -0,0 +1,16 @@
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <arpa/inet.h>
+#include "p2pnet.h"
+
+/* Create a UDP socket.
+ */
+int socket_udp4(void)
+{
+   int s;
+
+   s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
+
+   if (s == -1) return -1;
+   return s;
+}