Use upper case for "metasyntactic variables"

GNU typically uses all upper-case to refer to user-supplied text, as
opposed to the other common convention of angle brackets.

This patch fixes the cases of the latter that I found in help and
error text.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Kevin Buettner <kevinb@redhat.com>