busybox

hush: set default PS1/2 only if we interactive

"env - hush SCRIPT" invocation (that is, with empty environment)
should not show PS1/2 in "set" output.

function                                             old     new   delta
hush_main                                           1070    1075      +5

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