Re-add the notion of echo_prompt lost in the translation

Re-add the notion of echo_prompt lost in the translation

* src/keyboard.h (struct kboard): Replace echo_after_prompt with new
echo_prompt which contains the actual string.  Update all uses.

* src/keyboard.c (kset_echo_prompt): New function.
(echo_update): Add echo_prompt at the very beginning.
(read_char): Remove workaround for bug#19875, not needed any more.
(read_key_sequence): Set echo_prompt rather than echo_string (bug#21403).
(mark_kboards): Mark echo_prompt.