The actors, who both starred in the popular 1980s TV series “Magnum P.I.”, recently reunited on the set of the CBS procedural crime drama “Blue Bloods,” in which Selleck currently stars. A photo of the runion was shared by Selleck’s “Blue Bloods” co-star Donnie Wahlberg on Instagram Monday. In the snapshot, Selleck, 78, and Manetti,…