ash: support "set -m" monitor mode even when not interactive

[Steffen Nurpmeso]

Hello.

This came up on the dash list, and so i took over Jilles
Tjoelker's FreeBSD commit from 2014 to busybox ash.
Note i have no idea of what i am doing, but from testing it seems
to work; i have simply taken it over, which is a cleanup really.
(This is on top of my arithmetic patch but it should not really
interfere, .. in case linenumber shifts are seen.)

Ciao!

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)
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