Enforce strict response for data protection clause.

The - legally binding - data protection clause demands
an answer of "JA" to show voter agreement for vote
processing. Hence UseVote should only accept "JA"
(in upper or lower case), but not "DAFUER" or some
other synonyms.

* Add bdsg_confirm (regexp) to usevote.cfg
* Change check in uvvote.pl accordingly

Signed-off-by: Thomas Hochstein <thh@inter.net>