fix #2452

We have two basic example commands. But it’s not entirely finished because
there are some error checks that we don’t do.

ref #2521

fix #2469

Since the socket is now only created and managed whenever the connection is
being established, we only close the socket and if it was created (we use -1
to denote the fact that is not yet created, or has been closed) and we only
unmanage the socket if it is effectively managed.

fix #2529

It also happens to fix #2517 because this used to create buggy channels
named "auth" and stuf like that.

fix #2522

We want to call socket() with the parameters provided by getaddrinfo, so we
can’t addd the fd into the poller immediately. We need to wait the
connection attempt, and then the SocketHandler can call add_socket_handler
itself, if the connection succeeds, or is in progress.

Because the read handler may discover that the connection has been closed,
and then remove the socket from the poller. It that case it is no longer
valid to try to call the write handler (which may try to reconnect, but
since that socket is no longer managed, this is not OK).

Explain that the behaviour is otherwise undefined, in the comment.

fix #2524

Since “!” is also the separator between the nickname and the user hostname,
having “!” as the user mode (e.g. !nick!~some@host.bla) would cause the nick
to be empty. Now we skip it if it is a valid user mode indicator.