bus_generic_detach: Delete children after detaching them

[jhb]

Summary

This provides better semantics as a standalone DEVMETHOD for
device_attach as bus drivers should remove child devices they created
as part of detach cleanup. The implementation calls
bus_detach_children() first to permit child devices an opportunity to
veto the detach operation. If that succeeds, device_delete_children()
is used to delete the child devices.

This requires fixing various drivers that were deleting devices
explicitly (via a device_t pointer cached in the softc) after calling
bus_generic_detach to stop doing that and just rely on
bus_generic_detach to remove child devices.

[jhb]

I will bump __FreeBSD_version as well in this commit.

[imp]

Man page updates?

[jhb]

We have no manpages for this stuff. :( I've thought about writing a new manpage for bus_*_children and maybe some manpages for bus_generic_* (similar to the breakout I used on the slides for my talk in Dublin).

[imp]

In D47959#1093747, @jhb wrote:

We have no manpages for this stuff. :( I've thought about writing a new manpage for bus_*_children and maybe some manpages for bus_generic_* (similar to the breakout I used on the slides for my talk in Dublin).

That would be great. I'm cool with vague future promises..