All candidate commpaths added to localprefix #233

The selected commpath should be added to localprefix. Testing the multiple commpaths case (where some or all of the candidate commpaths could be unusable) flushed out a bug where all of the candidate commpaths were getting added to localprefix, including / (which was intended to be discarded by the selection process). That led to all files being considered local, and several obscure test failures.

On a related note: the cachepath should also be added to localprefix (to cover the pathological case of a user keeping local files in Spindle's cache).