newbus: Change attach failure behavior

In the rare case that we succeed in probing, but fail to attach, flip
the default to be to disable the
device. hw.bus.disable_failed_devices=false is no required to restore
the old behavior. The old behavior dates form a time when dynamic
control of devices wasn't yet present (devctl didn't exist). Now that
one can retry probe/attach the device with devctl, the default doesn't
make sense: The more desirable behaivor is to have stable device numbers
when one has several instances of the same device in a system (common
for NICs or HBAs).

Reviewed by: jhb (verbal)
Sponsored by: Netflix