All clients implement Kick, no need for a separate interface.

We now notice that a TLS certificate on disk has changed, and we
generate a self-signed certificate if none is found.

Thanks to Yacine.

Thanks to mscherer for the report.

We no longer risk deadlocking when sending actions, which in turn
enables pushing connections synchronously.

Doing this asynchronously was racy -- under extreme circumstances, a
client could be deleted before it was added.

If the writer gets congested, the whole group might be delayed.  Better
to drop the slow client in that case.

This simply indicates that the server closed the connection
before we received the close indication from the client.

Don't allow a user with a wildcard password entry to use the same
username as a user with a specific entry even at the same privilege
level.

A stat of a cached file is cheap, no need to micro-optimise.

We need to terminate all down connections synchronously, otherwise
we risk leaving open connections lying around.

We're already logging in the caller.

This reverts commit f4811e8c.

This turns out to be incorrect when a MediaEngine is attached.