bus: Add device_has_children predicate

[des]

Summary

Add a device_has_children() function which can be used to check if a
device has children without allocating a list of them which we aren't
going to use, or even counting them.

Also modify device_get_children() so it can be used to query the count
without allocating a list.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.

[des]

spell EFAULT correctly

[des]

update doxygen

[imp]

Why not

bool device_has_children(device_t dev) {return (!TAILQ_EMPTY(&dev->children))) }

[des]

I'm too used to thinking in terms of the least minimum diff to achieve the objective...

[des]

add device_has_children() like Warner suggested

[des]

tweak wording

[des]

reorder + style

[imp]

Looks like a lot of whitespace changes unrelated as noted.

sys/sys/bus.h

973

Are these intentional? If so, maybe a separate commit?

[des]

sys/sys/bus.h

973

Yes, they were intentional, and I can drop them.