Regression in 7.0BETA2: unionfs and cd9660

Hi,

I'm reporting a regression in recent RELENG_7, it was present (and I've
reported it) on BETA2, and I sadly confirm that it's also present in
BETA3. The problem is that cd9660 doesn't want to be the underlying
layer for unionfs.

How to repeat:

Boot a system with root on cd9660 (e.g. "LiveCD"), then create a
UFS-based md/mfs, then try to mount it via unionfs over (e.g.) /etc:

# <...make a ram-drive / memory file system on /tmp>
# mount_unionfs /tmp /etc
mount_unionfs: /etc: operation not supported by device

This worked ok with BETA1 and even -CURRENT versions, and I've located
the breaking point somewhere between:

#*default date=2007.11.07.00.00.00
#
#*default date=2007.11.08.00.00.00

I don't know if the date is local or UTC, but in any case I can't find a
lot of cvs commits that seem relevant. Maybe obrien's "ffs 'DIAGNOSTIC's
into INVARIANTS" or rwatson's work on MAC.