BUG/MINOR: ring: fix the way watchers are counted

BUG/MINOR: ring: fix the way watchers are counted

There are two problems with the way we count watchers on a ring:
  - the test for >=255 was accidently kept to 1 used during QA
  - if the producer deletes all data up to the reader's position
    and the reader is called, cannot write, and is called again,
    it will have a zero offset, causing it to initialize itself
    multiple times and each time adding a new refcount.

Let's change this and instead use ~0 as the initial offset as it's
not possible to have it as a valid one. We now store it into the
CLI context's size_t o0 instead of casting it to a void*.

No backport needed.