tabulated-list-init-header and glyphless-char-display

[Eli Zaretskii]

tabulated-list-init-header assumes without testing that u+25b2 and
u+25bc will be displayed as themselves.  But with various values of
glyphless-char-display-control, this may not be true.  Also, on text
terminals these characters generally cannot be displayed, which makes
the entire header line ugly, if not bug-like.

I'm not sure how to resolve this for text terminals.  Perhaps we
simply should not display these characters then.