fltk: Fix formatting escapes.

* fltk/main.cxx (fltk_cmd_handler): Fix calls to fl_message()
and fl_choice() functions.

The fl_message and fl_choice functions expect a format string as
their first argument; passing the message directly might cause a
crash (or worse) if the message happens to contain formatting
escape chars.

• GnuPG-bug-id: T4337
• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>