MINOR: ring/cli: support delimiting events with a trailing \0 on "show events"

MINOR: ring/cli: support delimiting events with a trailing \0 on "show events"

At the moment it is not supported to produce multi-line events on the
"show events" output, simply because the LF character is used as the
default end-of-event mark. However it could be convenient to produce
well-formatted multi-line events, e.g. in JSON or other formats. UNIX
utilities have already faced similar needs in the past and added
"-print0" to "find" and "-0" to "xargs" to mention that the delimiter
is the NUL character. This makes perfect sense since it's never present
in contents, so let's do exactly the same here.

Thus from now on, "show events <ring> -0" will delimit messages using
a \0 instead of a \n, permitting a better and safer encapsulation.