#
# call: $self->spawn_cmd(\<function>, [cb => sub{...}], [prefix => "cmd> "], [progress => 0|1], [args => [...]]);
sub spawn_cmd
#
# call: $self->spawn_cmd(\<function>, [cb => sub{...}], [prefix => "cmd> "], [progress => 0|1], [args => [...]]);
sub spawn_cmd