Mark unhittable lines

Add comments for the benefit of the lcov tool, marking
lines that cannot be hit.  Typically these are fall-back
protection arms that are already covered by earlier checks,
and so it's not worth taking out the unhittable code (in case
someone changes the code between the two places in future).

ares_library_init.c

@@ -111,7 +111,7 @@ int ares_library_init(int flags)
     {
       res = ares_win32_init();
       if (res != ARES_SUCCESS)
-        return res;
+        return res;  /* LCOV_EXCL_LINE: can't test Win32 init failure */
     }
 
   ares_init_flags = flags;