[fdo] Captcha added for subscription

[Tollef Fog Heen]

Hi all,

as some of you've probably noticed, list subscriptions have been broken
for a few days.  This was an attempt at handling a sustained
spam-through-signup attack.  Obviously, we needed something better and
list signups now require people to solve a captcha.  Hopefully this
should solve the problem.

Please let me know if there are any problems.

-- 
Tollef Fog Heen
UNIX is user friendly, it's just picky about who its friends are