Make detaching drivers from PCI devices more robust. While here, fix a
bug where a PCI device would be powered down if it failed to probe, but
not when its driver was detached (e.g. via kldunload).

• Add a new helper method resource_list_release_active() which forcefully releases any active resources of a specified type from a resource list.
• Add a bus_child_detached method for the PCI bus driver which forces any active resources to be released (and whines to the console if it finds any) and then powers the device down.
• Call pci_child_detached() if we fail to probe a device when a driver is kldloaded. This isn't perfect but can avoid leaking resources from a probe() routine in the kldload case.

Reviewed by: imp, brooks
MFC after: 1 month