Perform a terminal datapath stop before suspend and shutdown
callbacks, then drain the private configuration taskqueue before
entering low power.  Track power state independently of queue
ownership and prevent built-in admin, IOV, LED, and media-status
callbacks from accessing a suspended device.

Restore driver-specific state while the datapath remains stopped.
Initialize it exactly once on resume when the interface is
administratively up, and keep an administratively-down interface
stopped.  Roll back the driver when suspend or child suspension fails.

Add ifdi_power_prepare() for policy which must be established before
the terminal stop.  Use it to snapshot ixgbe(4) wake policy and preserve
X550EM PHY ordering, and remove the duplicate stop from aq(4).  Other
driver power-method wiring follows separately.

MFC after:      2 weeks