Added new smbmsg_t element: user_voted, used to pass around the results of

smb_voted_already(), e.g. to @-codes in a custom msg header.
New @-codes (e.g. for custom message headers):
@MSG_VOTED@
@MSG_UPVOTED@
@MSG_DOWNVOTED@
These all 3 act the same way: they display the PollAnswerChecked (checkmark)
if the user voted on the currently displayed message/poll, if the user up-voted
or if the user-downvoted. If the user did not vote, nothing is displayed.