MINOR: ring: add a generic CLI io_handler to dump a ring buffer

MINOR: ring: add a generic CLI io_handler to dump a ring buffer

The three functions (attach, IO handler, and release) are meant to be
called by any CLI command which requires to dump the contents of a ring
buffer. We do not implement anything generic to dump any ring buffer on
the CLI since it's meant to be used by other functionalities above.
However these functions deal with locking and everything so it's trivial
to embed them in other code.