Our modprobe is an ancient heritage from the past, probably dating to
a time where busybox's modprobe features were limited. There is no reason
at all to keep using our own version instead of the busybox one.
This commit also makes modules_scan 15% faster.
instead. At the same time, mount cdrom into /mnt/cdrom instead of
/newroot/mnt/cdrom (which is now just a bind mount), this avoids
losetup to expose unavailable paths inside the live system, breaking
mkfs.btrfs (next upstream version, which does silly things with
/proc/mounts).
Set DO_slowusb as default, and make setup_slowusb unset it if it cannot find
a usb-storage attached. This makes genkernel ramdisk adhere to "noslowusb",
makes the ramdisk only wait if there is a usb-storage attached and should fix
gentoo bug #359619.