The tmp rc script has much the same problem that the var does: it wants
to test if /tmp is writable, and mount a tmpfs if it's not. This means
that we actually want our zfs datasets mounted first, because we might
have a /tmp dataset that changes the story.

The ordering problem is particularly noticable with a r/o zfs root,
since the write test will fail and we'll mount a tmpfs that later gets
covered by our /tmp dataset. If that /tmp dataset inherited readonly,
then we're still in trouble.

This also fixes tmpmfs=yes, which would again get covered by a zfs
dataset with the existing ordering.