Modify COPY TO to emit carriage returns and newlines as backslash escapes

Modify COPY TO to emit carriage returns and newlines as backslash escapes
(backslash-r, backslash-n) for protection against newline-conversion
munging.  In future we will also tweak COPY FROM, but this part of the
change should be backwards-compatible.  Per pghackers discussion.
Also, update COPY reference page to describe the backslash conversions
more completely and accurately.