Move send_tun into tun.c as well.

Also remove a redundant include in checksum.c.

Change-Id: I0b8858343b1496f22904d3b316b6c435be0f648a
diff --git a/tun.c b/tun.c
index ef6b111..a1b7254 100644
--- a/tun.c
+++ b/tun.c
@@ -22,6 +22,9 @@
 #include <linux/if.h>
 #include <linux/if_tun.h>
 #include <sys/ioctl.h>
+#include <sys/uio.h>
+
+#include "clatd.h"
 
 /* function: tun_open
  * tries to open the tunnel device
@@ -60,3 +63,7 @@
   strcpy(dev, ifr.ifr_name);
   return 0;
 }
+
+void send_tun(int fd, clat_packet out, int iov_len) {
+  writev(fd, out, iov_len);
+}