Reject a copy EOF marker that has data ahead of it on the same line.

We have always documented that a copy EOF marker (\.) must appear
by itself on a line, and that is how psql interprets the rule.
However, the backend's actual COPY FROM logic only insists that
there not be data between the \. and the following newline.
Any data ahead of the \. is parsed as a final line of input.
It's hard to interpret this as anything but an ancient mistake
that we've faithfully carried forward.  Continuing to allow it
is not cost-free, since it could mask client-side bugs that
unnecessarily backslash-escape periods (and thereby risk
accidentally creating an EOF marker).  So, let's remove that
provision and throw error if the EOF marker isn't alone on its
line, matching what the documentation has said right along.
Adjust the relevant error messages to be clearer, too.

Discussion: https://postgr.es/m/ed659f37-a9dd-42a7-82b9-0da562cc4006@manitou-mail.org

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