<server>/state is now published directly to the <sever> topic level (the "state" topic goes away).

Moved <bbs-id>/node# topics to <bbs-id>/nodes/# (allows better wild-card subscription per MQTT standards).
Publishing total node count to <bbs-id>/nodes rather than <bbs-id>/node_count

Introduced "publish verbosity" control (defaults to "High") - if you want nice human readable (and sometimes redundant) topic messages, leave this set to "High". To reduce traffic, set to "Low". It's expected that purpose-built Synchronet/MQTT clients should work equally-well with either setting, but when using generic MQTT clients/browser, "high" verbosity is nice. The human-readable node status is only published when "high" verbosity is enabled.

Publishing the BBS name to the <bbs-id> topic, the instance hostname (as configured in sbbs.ini) to the <host> topic. Makes the hierarchy much more clear when using an MQTT browser like MQTT Explorer.

This is more consistent with how these events are logged in a BBS-common log file in data/*.log.

This change also restores the server abbreviation to the error log entries that used to be there until recently.

Each Synchronet server is now its own MQTT client. This means there's no
longer any MQTT logic in the Synchronet "hosts" (e.g. sbbscon.c, ctrl/*.cpp)
and none needed for SBBS NT services (they'll "just work" with MQTT).

This also means that just about everything (except for nodes, spam and hack)
is now published per-server (in the sbbs/BBS-ID/hostname/server/ topic branch)
and if you want aggregated totals or client lists, you'll have to do that in
your own MQTT client or dashboard.

I also removed the publishing of thread_count and socket_count topics as
they weren't universally supported across all servers and are of questionable
value. They can be added back later if determined to be useful.

Should fix the new crash issues reported by Retros via #synchronet on irc.synchro.net

warning: passing argument 2 of ‘mosquitto_message_callback_set’ from incompatible pointer type

Now support subscriptions (e.g. recycle topics, node input topics) in SBBSCTRL.
This required a lot of search/replace and fun with circular struct pointers.

The only difference in the data/scheme is that the "error" topic (error log) is now under each server rather than each host. I don't *think* there are any other changes from the MQTT consumer side.

Still not done: subscribing (e.g. support for recycle or node-spy-input via MQTT) and NT services support.

This change also includes a cool feature that will prompt the sysop if there's a timeout (30 seconds) while waiting for servers to shutdown gracefully and giving the sysop the option to abort (Cancel) the wait (and shutdown ungracefully) or continue the wait (OK).

By moving mosquitto_will_set() call into mqtt_connect().

I'm still having the issue that the LWT is being published after mosquitto_disconnect(), even though I set the "force" parameter to false in the call to mqtt_loop_stop()

This can be used to clear a 'retained' topic (of any type).

(minimum severity of log messages that will be published via MQTT)

publish QOS still defaults to 0 (for now at least), while
subscribe QOS defaults to 2.

Default MQTT version is 5
Default username, password are block. Up to 255 chars each.

Requires libmosquitto (e.g. install mosquitto-dev pkg) and set
in ctrl/main.ini:
[mqtt]
Enabled=true
broker_addr=127.0.0.1
broker_port=1883
qos=0
keepalive=10

Next up: Windows support