MINOR: applet: adopt the wait list entry from the CLI

MINOR: applet: adopt the wait list entry from the CLI

A few fields, including a generic list entry, were added to the CLI context
by commit 300decc8d9 ("MINOR: cli: extend the CLI context with a list and
two offsets"). It turns out that the list entry (l0) is solely used to
consult rings and that the generic ring_write() code is restricted to a
consumer on the CLI due to this, which was not the initial intent. Let's
make it a general purpose wait_entry field that is properly initialized
during appctx_init(). This will allow any applet to wait on a ring, not
just the CLI.