dump after ddb sync command

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I just experienced a panic on our (4.0/amd64) file server, which dropped into 
ddb.
I issued a "sync" command, which synced the disks, unmounted them (which I've 
never noticed before) and then went on to take a dump.
Is that expected behaviour? I didn't want to take the dump and found no way to 
interrupt it short of pressing the red button.
On reboot, the file systems were marked clean; but savecore tried to fetch the 
dump I had aborted (I interrupted that).
Shouldn't the dump be marked valid only after it was fully done?