Rollback rev 9253 from 3.1

Rollback rev 9253 from 3.1

This change to StoreIO overlooked the signedness of the StoreIO* length
parameter. It may have resulted in objects that should not have been
store making their way into the cache.

Caches created by 3.1.0.2 and earlier releases are known to contain many
invalid entries. Whether or not these entries are fatal to Squid is still
unknown. It is currently expected that they will be erased properly, but
cause a lot of cache.log warnings while that is happening.
It may be worth purging caches on upgrade out of these versions.