Allow whitspace to appear between device name and unit number in loaderdev.
This allows the same value to be used in u-boot commands and the loaderdev
env var that gets passed to ubldr, for example 'fatload mmc 1 ${bootfile}".
Switch zfsboot installer force 4K option to use vfs.zfs.min_auto_ashift=12

Previously we used gnop(8) to fake 4K sector size disks but ZFS now has a
sysctl to control this when creating new top level vdev's so use that
instead.

Differential Revision:	D566
MFC after:	1 month
Sponsored by:	Multiplay