Add a configurable sleep timer to pre_mountroot.

Add a configurable sleep timer to pre_mountroot.

Add a new `/etc/default/zfs` option:

* `ZFS_INITRD_PRE_MOUNTROOT_SLEEP`, which wastes an arbitrary amount
of time in the initrd environment before wait_for_udev is called.

This option is primarily a kludge for bad mpt2sas hardware that can go dumb
during system initialization. Thirty seconds should be enough for most
affected computers, but five minutes might be required in the worst case.

Closes: dajhorn/pkg-zfs#71