hush: make it possible to have interactive shell on non-ctty.

hush: make it possible to have interactive shell on non-ctty.
 init=/bin/hush: shows prompt, history works, etc.

function                                             old     new   delta
hush_main                                            888     925     +37
block_signals                                        139     152     +13
builtin_fg_bg                                        284     293      +9
checkjobs_and_fg_shell                                35      41      +6
sigexit                                               65      66      +1
reset_traps_to_defaults                              165     164      -1
parse_stream                                        2200    2184     -16
run_list                                            2502    2475     -27
getpgid                                               35       -     -35
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(add/remove: 0/1 grow/shrink: 5/3 up/down: 66/-79)            Total: -13 bytes