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.