Octave 7 compatibility fix: suppress spurious warnings

In this version of Octave, “warning on” really enables all warnings (while this
was not the case in Octave 6, suprisingly; I think this used to be different
with even older Octave releases).

We therefore need to explicitly disable the Octave:array-as-logical warnings,
that is triggered by some internal Octave functions.

We also need to be careful to call “warning_config” instead of “warning on”
when restoring warnings to their default state.