Code names. Hidden identities. Surveillance cameras. Until 2010, when ‘‘The Catcher In the Rye’’ novelist died at age 91, only a handful of people were fully aware of what he was up to. Even now, with the release date of the film ‘‘Salinger’’ less than three weeks away, little is known about a production that draws upon more than 100 interviews and a trove of documents and rare photographs, and that promises many revelations about an author who still fascinates millions.