Make tinc --batch --force join enable the tinc-up script.

Make tinc --batch --force join enable the tinc-up script.

The expected behavior of --batch --force is that all parameters in the
invitation are accepted, whether unsafe or not. Unsafe variables were
already accepted with --force in commit 061362d2f, this commit ensures
the generated tinc-up script is enabled as well.

Fixes #196 on GitHub.