Unix-like OS with job control. The progress ticker is suppressed
for a command like "ngspice some_circuit.cir &" so the terminal
remains usuable while the simulation runs.
That makes it possible to push ngspice into the background while
plot windows are open: the windows remain responsive.
Discussion on ngspice-users, starting 2023-09-01,
title: "`quit` in Interactive Mode."
Also restore terminal state when forcing exit with SIGINT (control-C).