zfsboot patch for /usr

[Roger Marquis]

> I no nothing about this script but if you want /usr/local as ZFS
> filesystem, then you need to create parent (/usr in this case) and you
> can use property canmount=off plus different 'mountpoint' (for example
> /mnt/usr) to not mount /usr over existing directory on root filesystem.

Exactly, but in our case we don't want a separate partition for /usr (never
understood why anyone would since 1GB drives became affordable).  As a beadm
user I'm also assuming a rollback-able boot image would need to include /usr
in the root filesystem.

To do this in bsdinstall/scripts/zfsboot requires a 'mkdir' instead of 'zfs
create'.  Looks like it might be a matter of copying the $MKDIR_P around line
1125 but am testing that now.

Roger