libpq failed to cope with COPY FROM STDIN if the command was issued

libpq failed to cope with COPY FROM STDIN if the command was issued
via extended query protocol, because it sends Sync right after Execute
without realizing that the command to be executed is COPY.  There seems
to be no reasonable way for it to realize that, either, so the best fix
seems to be to make the backend ignore Sync during copy-in mode.  Bit of
a wart on the protocol, but little alternative.  Also, libpq must send
another Sync after terminating the COPY, if the command was issued via
Execute.