Fernanda Eberstadt profile of Juan Goytisolo, Spanish novelist who has lived for much of last 25 years in Marrakesh, Morocco, indulging his passion for Muslim culture and predilection for working-class Arab men; Goytisolo has written against tyranny of Arab governments, which he blames for radicalization of Islamic youth, and also against neglect of Europe's Islamic minorities and pernicious influence of Christian evangelism on American foreign policy; nevertheless sees seeds of modernity in Arab world and hopes that radical Islamist parties may be tempered and matured by partaking in national governments; photos (L)