What’s an esquire?” asks the aunt of Roman Israel’s (Denzel Washington) jailed client-to-be. She wants to hire a lawyer for her wrongly accused nephew and trusts Israel because he says what’s on his mind.“Slightly above a gentleman, slightly below a knight,” explains the frumpy legal savant with the lopsided Afro and aviator glasses in the film “Roman J. Israel, Esq.”Quixotic Israel has been tilting at windmills since his days in the 1970s as an activist at University of …