[Tails-dev] Made New Identity work again, please review

I believe I have fixed the regression described in ticket #6383. When
access to Tor's control port was restricted (to prevent GETINFO address),
Torbutton could no longer do "New Identity". I have created a filtering
proxy for the control port, that only allows SIGNAL NEWNYM. This is enough
to make "New Identity" work again as expected.

Branch "winterfairy:bugfix/torbutton-new-identity" in my Tails repo (based
on devel).

The design documentation is also updated, and the issue removed from
"known issues".

Well tested during development, both in normal operation and failure
conditions (real and faked). Also tested by patching a Tails 0.22.1
installation, and ensuring it works live (circuits gets replaced as shown
in Vidalia's network map).

Please review and test it, and merge it is it looks fine. If you believe
something could be improved, or should be done differently, please say.

(I did not use Whonix existing implementation, because I felt very uneasy
about running a potentially security critical application as a shell
script. Also, it required dependencies not currently in Tails, it seems,
and I have not the knowledge to review shell scripts from a security point
of view.)