gcc10 builds with -fno-common by default. This mean you can't define
a global variable with the same name twice.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1799680

"Multicast" is perhaps a more conventional name for the concept.
"One-shot" is the conventional name for how the event is (currently)
scheduled.

This makes external UI behave consistenly with TUI w.r.t resizes.
Which will be needed anyway as TUI will use the external UI protocol
soon.

    ../src/nvim/event/rstream.c:119:44: warning: format specifies type 'void *' but the argument has type 'Stream *' (aka 'struct stream *') [-Wformat-pedantic]
          DLOG("Closing Stream (%p): %s (%s)", stream,
                                ~~             ^~~~~~
    ../src/nvim/event/stream.c:95:30: warning: format specifies type 'void *' but the argument has type 'Stream *' (aka 'struct stream *') [-Wformat-pedantic]
      DLOG("closing Stream: %p", stream);
                            ~~   ^~~~~~
    ../src/nvim/msgpack_rpc/channel.c:71:72: warning: format specifies type 'void *' but the argument has type 'Stream *' (aka 'struct stream *') [-Wformat-pedantic]
        DLOG("rpc ch %" PRIu64 " in-stream=%p out-stream=%p", channel->id, in, out);
                                           ~~                              ^~
    ../src/nvim/msgpack_rpc/channel.c:71:76: warning: format specifies type 'void *' but the argument has type 'Stream *' (aka 'struct stream *') [-Wformat-pedantic]
        DLOG("rpc ch %" PRIu64 " in-stream=%p out-stream=%p", channel->id, in, out);
                                                         ~~                    ^~~
    ../src/nvim/msgpack_rpc/channel.c:226:28: warning: format specifies type 'void *' but the argument has type 'Stream *' (aka 'struct stream *') [-Wformat-pedantic]
           channel->id, count, stream);
                               ^~~~~~

We already do this for _invalid_ async requests #9300.
Now we also do it for failed invocation of valid requests.