patch: squid-2.6 rebuilds

Hiya,

Here's my initial patch to make things slightly saner during cachedir
rebuilds.

Its resulted in a correct-looking rebuild from a cache w/ COSS+AUFS which
crashed during rebuild. I haven't contacted the owner of the bugzilla
entry to see if this patch fixes his particular problem but I do believe
they're the same.

Comments please! I'd like to commit a fix to this issue as soon as is possible.

Adrian