bsdinstall on zfs: create dataset for /home rather than /usr/home

[karels]

Summary

Now that pw (hence adduser and the initial install) use /home for
user home directories rather than /usr/home, create a dataset for
/home rather than /usr/home. Update the man page to match.

[rgrimes]

You could add the hier(7) man page update to this review since this change is what effects that man page too.

[karels]

Adding @markj, at least for context.

[markj]

This looks like it does what it's supposed to do.