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

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.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47673

(cherry picked from commit 46297859a74563dde6fc5bff9f9ecded9fb61ba6)

bus_generic_(probe|attach|detach) will not be changed in stable/14,
but providing the new APIs in stable/14 permits drivers to use the new
APIs.