BUG/MEDIUM: cli/ring: Properly handle shutdown in "show event" I/O handler

BUG/MEDIUM: cli/ring: Properly handle shutdown in "show event" I/O handler

The commit 03dc54d802 ("BUG/MINOR: ring: Fix I/O handler of "show event"
command to not rely on the SC") introduced a regression. By removing
dependencies on the SC, a test to detect client shutdowns was removed. So
now, the CLI applet is no longer released when the client shut the
connection during a "show event -w".

So of course, we should not use the SC to detect the shutdowns. But the SE
must be used insteead.

It is a 3.2-specific issue, so no backport needed.