Viacom CEO Philippe Dauman’s battle with Chairman Emeritus Sumner Redstone could hurt the company’s ability to borrow: Standard & Poor’s Global Ratings today lowered its assessment of Viacom’s management and governance to “fair” from “satisfactory.” It reaffirmed the BBB- rating for the company’s debt but said that if the drama continues, it “could lead to a reassessment” of Viacom’s creditworthiness. Dauman’s suing to have Redstone, who turned 93 today, declared…