Add a new rescan method to the bus interface.

[jhb]

Summary

Add a new rescan method to the bus interface.

The BUS_RESCAN() method rescans a single bus device checking for devices
that have been added or removed from the bus. A new 'rescan' command is
added to devctl(8) to trigger a rescan.

Test Plan

• Used 'devctl rescan' as a poor man's pci-e hotplug implementation in conjunction with a followup commit to implement the rescan method for the PCI bus driver.