Thoma Bravo-backed Proofpoint Inc. is in talks to acquire cybersecurity company Varonis Systems Inc., which would be one of the biggest technology take-privates since the steep selloff in software stocks earlier this year, according to people familiar with the matter.Proofpoint is negotiating the terms of a potential deal with Varonis, which has been working with advisers to field takeover interest, said the people, asking not to be identified because the information is private. A transaction co...