udev: Set "deactivated" USB configuration on iOS devices by default

With the introduction of iOS 7, devices should start of in the "deactivated"
USB configuration "0" by default to not trigger a trust dialog.
Once the devices are identified, usbmuxd will set the correct USB configuration
itself and immediately run the preflight worker code to ensure a trust
relationship with the host is established. This change fixes the trust dialog
from appearing during hotplug of paired devices. The last remaining issue is
that current kernel code still sets the USB configuration to "1" by default
before the udev rules and thus causes multiple connection beeps.
preflight: Add lockdownd prefix to name of set_untrusted_host_buid()
preflight: Make device visible during trust dialog pairing and handle unplug

The usbmuxd implementation on Win/OS X does allow enumerating and accessing the
device during the "trust dialog" pairing process. We now also exit the waiting
loop during unplugging of a device while waiting for the trust dialog to be
dismissed.
preflight: Add messages to indicate start and finish of preflight process
usb: Output message to show USB configuration change