Ticket #3001: fix read and update of subshell prompt.

The bug was introduced in e35f044ccdd41922f925c99e6d50930ea8c7c47e.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(strip_ctrl_codes): type accuracy.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Cleanup: remove unused 'prompt_pos' variable.

(do_subshell_chdir): remove 'reset_prompt' argument because of removed
'prompt_pos'.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>