Mark David Marshall Herskovitz, co-creator and executive producer of the seminal late-1980s and early-’90s primetime drama “Thirtysomething,” and currently president emeritus of the Producers Guild of America, has put a $6 million price tag on his luxury ranch in the rugged Monte Nido area in the Santa Monica Mountains, a winding 7½-mile drive north of the Malibu Pier.The four-time Emmy winner, an Oscar nominee for the 2000 film “Traffic,” paid $4.1 million for the secluded manse in 2009, right