QP_stream: char *prompt
A prompt string is a character string that will be written out when
a tty stream (a stream with QP_DELIM_TTY format) reads input
at the beginning of a line. The prompt string is stored
in this field. This field is only used for an input tty stream.
The prompt string will not be written out if there are no output
tty streams connected to the same tty as the input tty stream
(see fli-ios-tty). A prompt string of a stream may be changed after the
stream is created through prompt/[2,3] or by assigning a new
character string to this field directly.