This patch adds support for TLS-wrapped submission connections.
Instead of clients establishing a connection over plain text and then
using STARTTLS to switch over a TLS connection, this new mode allows the
clients to connect directly over TLS, like it's done in HTTPS.
This is not an official standard yet, but it's reasonably common in
practice, and provides some advantages over the traditional submission
port.
The default port is 465, commonly used for this; chasquid defaults to
systemd file descriptor passing as for the other protocols (for now).
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