THE HAGUE, Netherlands — Police arrested 90 protesters Saturday as a traditional Dutch celebration of the arrival of St. Nicholas was disturbed by demonstrators who say his faithful sidekick, “Black Pete,” is a racist caricature, and by supporters of the popular children’s figure. In some of the worst scenes of unrest in the increasingly acrimonious debate about Black Pete, opponents scuffled with police on the historic market place of the central city of Gouda as thousands of children welcomed St.