Addition of new fields to the usb_devinfo struct to enable the new
and shiny usbd daemon to handle events.

usb_port.h:

• Add a macro to retrieve the unit number from a USBBASEDEVICE

usb.h, usb_subr.c:

• Add fields to the device_info struct.

usb_subr.c:

• Fill in the new fields.
• Remove the notification of the event up a bit to make sure all the information is still available to fill the usb_devinfo struct.

This requires recompilation of usbdevs (src/usr.sbin/usbdevs) and the
ezdownload/ezupload (ports/misc/ezload) utilities in any case.