client: At checkout, catch UUID collisions with other checkouts
client: At checkout, catch UUID collisions with other checkouts

- At checkout, after obtaining the parcel.cfg, make sure there is not
already a parcel directory matching the obtained UUID.  This could indicate
a UUID collision or a parceldir which doesn't contain a valid parcel.cfg.
In either case, we errexit rather than clobbering the existing parcel
directory.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr