Allow device hints to wire the unit numbers of devices.

• An "at" hint now reserves a device name.
• A new BUS_HINT_DEVICE_UNIT method is added to the bus interface. When determining the unit number of a device, this method is invoked to let the bus driver specify the unit of a device given a specific devclass. This is the only way a device can be given a name reserved via an "at" hint.
• Implement BUS_HINT_DEVICE_UNIT() for the acpi(4) and isa(4) bus drivers. Both of these busses implement this by comparing the resources for a given hint device with the resources enumerated by ACPI/PnPBIOS and wire a unit if the hint resources are a subset of the "real" resources.
• Use bus_hinted_children() for adding hinted devices on isa(4) busses now instead of doing it by hand.
• Remove the unit kludging from sio(4) as it is no longer necessary.

Prodding from: peter, imp
OK'd by: marcel
MFC after: 1 month