Move tun functions to a new tun.c.

Change-Id: I4910b6cfca0bbf6bde2d1348f43d3131a1212399
diff --git a/Android.mk b/Android.mk
index a640e06..819c86f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1,7 +1,7 @@
 LOCAL_PATH:= $(call my-dir)
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES:=clatd.c dump.c checksum.c translate.c icmp.c ipv4.c ipv6.c config.c dns64.c logging.c getaddr.c netlink_callbacks.c netlink_msg.c setif.c mtu.c
+LOCAL_SRC_FILES:=clatd.c dump.c checksum.c translate.c icmp.c ipv4.c ipv6.c config.c dns64.c logging.c getaddr.c netlink_callbacks.c netlink_msg.c setif.c mtu.c tun.c
 
 LOCAL_CFLAGS := -Wall -Werror -Wunused-parameter
 LOCAL_C_INCLUDES := external/libnl/include bionic/libc/dns/include