MFC r355010: Make CAM use root_mount_hold_token() to delay boot.

Before this change CAM used config_intrhook_establish() for this purpose,
but that approach does not allow to delay it again after releasing once.

USB stack uses root_mount_hold() to delay boot until bus scan is complete.
But once it is, CAM had no time to scan SCSI bus, registered by umass(4),
if it already done other scans and called config_intrhook_disestablish().
The new approach makes it work smooth, assuming the USB device is found
during the initial bus scan. Devices appearing on USB bus later may still
require setting kern.cam.boot_delay, but hopefully those are minority.