When usb_event_thread() first starts, wait significantly longer

When usb_event_thread() first starts, wait significantly longer
before starting exploring (4 seconds), and extend the wait period
if new USB buses are attached while waiting.

This works around a problem seen when there is more than one EHCI
controller in the system and you kldload usb.ko after the system
has booted. The problem is that usb.ko contains 3 separate PCI
drivers which get initialised one by one (uhci, ohci, ehci), and
when each driver is initialised, all PCI buses are re-probed after
just the addition of that driver. This means that there can be a
significant delay between the attaching of a companion controller
and the subsequent EHCI attach, so it is possible for the companion
controller's USB 1.x bus to be scanned before the EHCI driver gets
a chance to check if there is really a USB 2.x device connected.