Check correct voting in ballot.

It's possible that ballots get mixed up when
a voter is participating in more than one
voting and sending the ballots manually (by
copying and pasting them into his mail client).

UseVote could - and should! - check that the
ballot belongs to the current voting by comparing
the ballot introduction ("WAHLSCHEIN fuer ...")
with the votename and throw an error if they
don't match.

* Add ballotintro (default: WAHLSCHEIN fuer) and
  tpl_wrong_voting (default: wrong-voting) to
  usevote.cfg
* Add messages to messages.cfg
* Modify ballot and ballot-personal templates
  accordingly
* Add check to uvvote.pl
* Add error handling to UVmenu.pm
* Change UVmenu::menu call in uvcfv.pl due
  to new parameter ($voting)

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