Tunnel inbound mail to the device - #762

redirect terminates port 25 and forwards each message down the frp tunnel, so
the device has to ask for a proxy on the port it was allocated. frpc gains a
tcp proxy alongside the https one, named <domain>-smtp to match the suffix
redirect strips when it attributes traffic and authorises the port.

The tunnel is no longer all or nothing. Enable and Disable became Apply, which
renders whichever proxies the settings call for: web when the traffic relay is
on, mail when the mail relay is on and a port has been handed out, either
without the other. That matters because the common case for mail is a device
with a public address whose isp blocks 25 outbound and inbound; it needs a
tunnel for mail alone and had no way to get one.

The port comes back on the domain update, which already runs on both paths:
the access page calls Update and the mail relay switch calls Sync. Update
applies the tunnel a second time afterwards, since the port only exists once
redirect has answered, and without that a device turning the mail relay on
would wait for some unrelated change before its mail proxy appeared. A
response that cannot be read is logged and ignored rather than failing the
update, because the address change has already happened and undoing it over a
port would be worse.

Bringing the tunnel up now waits for every proxy it asked for, not just the
web one, so a half connected tunnel is a failure rather than a device that
looks fine and quietly receives no mail.

Local port 10025 is the contract with the mail app: postfix listens there for
mail arriving through the tunnel, separately from its own port 25.

Every device was allocated a port on the relay so mailin had something to
dial, which meant a range with a ceiling, ports that were never reclaimed, and
the number travelling back through the domain update into the device's config.

frp routes the web proxy by name over one shared port already. tcpmux does the
same for raw tcp, matching the name in an http CONNECT that mailin sends
itself, so the mail proxy can share a port the same way. The device asks for
its tunnel by its own domain, which is also what the auth plugin already
checks for the web proxy.

That removes the smtp port from the update response, from the device config
and from this side entirely.