Unworkable column delimiter characters for COPY

Currently, copy.c rejects newline, carriage return, and backslash as
settings for the column delimiter character (in non-CSV mode).  These
all seem necessary to avoid confusion.  However, I just noticed that the
letters r, n, t, etc would also not work: on output, data characters
matching such a delimiter would get escaped as \r, \n, etc, which on
input would be read as C-style control characters.

I think at minimum we need to forbid b, f, n, r, t, v, which are the
control character representations currently recognized by COPY.
But I'm tempted to make it reject all 26 lower-case ASCII letters,
as a form of future-proofing.  Thoughts?
        regards, tom lane