First Advent   As I walked through the streets of our city on this first Sunday of Advent, I noticed him at once. A man slumped on a park bench, surrounded by a few paper bags and a shopping trolley — a home on four squeaking wheels. He looked the way people expect someone in his position to look gaunt, exhausted, weather-beaten, with tangled hair, a wild beard and a jacket that had survived too many winters.   My first thought was the usual one — and I was ashamed of it almo