Change: Add separate setting for server sent commands per frame limit (#11023)

Set a higher default value for this setting.
Use the higher of this and existing commands per frame limit
setting for server-originating commands, e.g. GS.

This is to support the GSAsyncMode class.
This also avoids undue throttling when more than one
script is in operation (e.g. AIs).

Fix: no_http_content_downloads and use_relay_service as private settings (#10762)

Basically, we don't need to know those values when people send in
crash reports.

Change: remove the ability to control "max spectators" (#9466)

Soon we will make "join game" join the game as spectator first,
so limiting the amount of spectators makes no sense anymore in
that context. Not sure it ever did make sense.

Feature: allow the use of TURN to connect client and server together

TURN is a last resort, used only if all other methods failed.
TURN is a relay approach to connect client and server together, where
openttd.org (by default) is the middleman.

It is very unlikely either the client or server cannot connect to
the STUN server, as they are both already connected to the Game
Coordinator. But in the odd case it does fail, estabilishing the
connection fails without any further possibility to recover.

Add: allow setting your server visibility to "invite-only" (#9434)

In this mode you do register to the Game Coordinator, but your
server will not show up in the public server listing. You can give
your friends the invite code of the server with which they can
join.

Change: move sensitive information to secrets.cfg and private information to private.cfg

We often ask people for their openttd.cfg, which now includes their
passwords, usernames, etc. It is easy for people to overlook this,
unwillingly sharing information they shouldn't.

By splitting this information over either private.cfg or secrets.cfg,
we make it more obvious they shouldn't be sharing those files, and
hint to what is inside them.

Codechange: move network-related settings out of settings.ini

This to prepare the code to split up network-related settings
into private / secrets / generic.