pinentry-tty: make confirm actions work

* tty/pinentry-tty.c: treat the situation where no PIN is requested
  and one_button is not set as a confirmation prompt.

When user confirmation is requested on a dumb terminal, we use the
value of the "OK" button followed with [y/N]? as the confirmation
prompt. User typing is echoed as normal, since a confirmation prompt
is not a password entry.