Support get_single_char/1 in PDT console.
This predicate modifies user_input and user_output
as follows:
- If single-char mode is enabled, write "ESC s" over the user output
and then wait for two characters, returning the first.
- Output is changed to emit ESC as ESC ESC.
This protocol was designed for PDT by Lukas Degener. The original
implementation was partly in Prolog. This is a full C implementation,
both for speed.