Fix help message for BouncerServ's network update

It was missing the network name as first argument.
readme: update mailing list link
doc: fix typo in man page
config: make http-origin directive overwrite previous list

Let's be on the safe side and assume the user doesn't meant the union of
all directive values.
Add accept-proxy-ip config directive

This allows to set the list of IPs allowed to act as a proxy. This is
only used for WebSockets right now, but will be expanded to TCP as well
once the PROXY protocol is supported.
Change unix:// to irc+unix://

When Unix socket support will be added for listeners, unix:// will be
ambiguous. It won't be clear whether to setup an IRC server, or some
other kind of server (e.g. identd).

unix:// is still recognized to avoid breaking existing DBs.
doc: add example config file in man page
readme: mention the znc-import tool
readme: expand usage
doc: mention SASL EXTERNAL in the CertFP docs
Rename `certfp reset` to `sasl reset`

And make it reset all SASL credentials.
Add a `sasl set-plain` command

This allows to manually set the SASL credentials for a network.
doc: document the network update command

References: https://todo.sr.ht/~emersion/soju/12
Prevent error handler from falling through in user.run
Add unix as supported upstream URL scheme to service
contrib/znc-import: new utility

Allows populating the soju database from a ZNC config file.
Fix deadlock in DB.Close

This method was calling itself, instead of the underlying SQLite
database's Close method.
Broadcast unhandled messages to downstream connections

In case labelled-response isn't supported, broadcast unhandled messages
to all downstream connections. That's better than silently dropping the