A:

• There really isn't such a thing as "a vote to abstain." An abstention is actually no vote at all.
• The chair should call for affirmative and negative votes, but not for abstentions.
• If there is a requirement for unanimity (i.e., a "black ball" vote), it's usually that those present and voting be unanimous, in which case abstentions would not count.
• No one should be required to vote. There are several legitimate reasons to abstain. See RONR p. 402. Besides, all one need do to "abstain" if voting is required is leave the meeting.