BORDER CROSSING Set in the north of England, Pat Barker's new novel portrays a child psychiatrist who rescues a man from drowning one day while walking on a beach in Northumberland.   Uncannily, he recognizes the man; it's Danny Miller, a child murderer at whose trial he once gave evidence.  Since the trial, he has reconsidered that evidence and found it lacking.  Now he confronts the man whose altered fate may be his responsibility.