basic/terminal-util: fix output of files without a final newline

basic/terminal-util: fix output of files without a final newline

If the main config file or one of the drop-ins did not have the final newline,
there would be no seperating empty line (or if this was the last file
displayed, our own output would end without the final newline, possibly running
into the subsequent prompt or such). copy_bytes() does not know anything about
lines, so let's just use a normal loop with read_line() and puts().