Add support for disabling prefix key handling

To disable prefix key input handing simply set prefix-ctrl-key to
'none'.

Based on original patch from Sebastian Krahmer.

src/options.h

@@ -59,6 +59,7 @@ struct option_t
     bool hex_mode;
     unsigned char prefix_code;
     unsigned char prefix_key;
+    bool prefix_enabled;
     bool response_wait;
     int response_timeout;
     bool mute;