(set_clipboard_data): Terminate the text with a null

(set_clipboard_data): Terminate the text with a null
character.  Don't allow to put binary data into the clipboard.
Return zero in case of success, 1 or 2 otherwise.
(get_clipboard_data): Only bail out if the null character is in
the last 32-byte chunk of clipboard data
(Fw16_set_clipboard_data): Make ok and put_status be unsigned.  If
they save binary data, print a message in the echo area saying the
text was not put into the clipboard.