Modify COPY FROM to match the null-value string against the column value

Modify COPY FROM to match the null-value string against the column value
before it is de-backslashed, not after.  This allows the null string \N
to be reliably distinguished from the data value \N (which must be
represented as \\N).  Per bug report from Manfred Koizar ... but it's
amazing this hasn't been reported before ...
Also, be consistent about encoding conversion for null string: the form
specified in the command is in the server encoding, but what is sent
to/from client must be in client encoding.  This never worked quite
right before either.