Don't use 0 as null pointer constant, part 1

Use NULL instead of 0, for clarity.  Except in pointer comparisons;
leave that to the next two commits.

src/lib/common/nat.c

@@ -50,7 +50,7 @@ cname(natid n)
     struct natstr *np;
 
     if ((np = getnatp(n)) == 0)
-	return 0;
+	return NULL;
     return np->nat_cnam;
 }