Move main() out of clatd.c.

This allows us to unit test methods that are in clatd.c.
Also simplify the build file, adding a defaults stanza and
grouping files in a filegroup so that both the code and the unit
test have the same source files.

Test: atest clatd_test
Test: builds, boots, 464xlat works
Change-Id: I544c3ee846abd3e38b80a2d9a4db5497fd9beb0c
diff --git a/tun.h b/tun.h
index bcdd10e..f0449b9 100644
--- a/tun.h
+++ b/tun.h
@@ -20,7 +20,7 @@
 
 #include <linux/if.h>
 
-#include "clatd.h"
+#include "common.h"
 #include "ring.h"
 
 struct tun_data {