For nearly two decades, the world at large knew precisely where to find the acclaimed guitarist Kevin Eubanks. For five nights a week, Eubanks was cranking up the band and issuing steamy solos as music director on NBC’s “Tonight Show with Jay Leno.” In spring of 2010, after 18 years on the job, Eubanks decided to move on, although retirement was the last thing on his mind. That much is clearly stated in potent musical terms on Zen Food, his first recording for the Mack Avenue label.