We want to be able to distinguish between connections for SMTP and connections
for submission, so we can make different policy decisions.
To do that, we first make the configuration aware of the different kinds of
addresses. This is done in this patch in a backwards-incompatible way, but at
this point in time it is ok to do so.
Then, we extend systemd's socket passing library to support socket naming, so
we can tell the different sockets apart. This is done via the
LISTEN_FDNAMES/FileDescriptorName mechanism.
And finally we make the server and connection types aware of the socket mode.
This patch introduces a basic on disk configuration, comprised of a main
configuration file and per-domain directories.
It's still not complete, but will be extended in subsequent patches.
