Use idempotent operations for one pass initialization with no retries

One-pass counter initialization with no need for retries.

Created with create-or-open (O_CREAT, non-EXCL).
Initialized to binary zero with ftruncate
Converted to textual zero with compare-and-swap

These idempotent operations may be performed by any number of racing odos with any combination of interleaving and interruptions. They will silently do nothing if already performed by another odo.