hush: improve prompt in nested {}s, remove unused in_str->promptme member

hush: improve prompt in nested {}s, remove unused in_str->promptme member

function                                             old     new   delta
setup_string_in_str                                   29      38      +9
parse_and_run_stream                                  79      88      +9
setup_file_in_str                                     32      39      +7
parse_stream                                        2430    2422      -8
file_get                                             262     235     -27
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 25/-35)            Total: -10 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>