new-bus: Add bus_(identify|attach|detach)_children

[jhb]

Summary

These correspond to the current implementations of
bus_generic_(probe|attach|detach) but with more accurate names and
semantics. The intention is to deprecate bus_generic_(probe|attach)
and reimplement bus_generic_detach in a future commit.

[jhb]

FYI: I will bump __FreeBSD_version after committing this (and before any the other changes).

[imp]

Why not with? Either is fine though

[jhb]

I guess with is fine, but now I'll wait until stabweek is over.

[imp]

In D47673#1088578, @jhb wrote:

I guess with is fine, but now I'll wait until stabweek is over.

Fair enough... This change looks safe, but hasn't been widely tested as safe, so waiting is a good idea to give it 3ish weeks to cook before the next stabweek.