hush: fix "unset PS1/PS2", and put them into initial variable set

hush: fix "unset PS1/PS2", and put them into initial variable set

"unset PS1/PS2" causes prompts to be empty strings

function                                             old     new   delta
hush_main                                           1031    1089     +58
goto_new_line                                         27      33      +6
fgetc_interactive                                    244     245      +1
unset_local_var                                      155     149      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 65/-6)              Total: 59 bytes

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