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 does various minor style and simplification cleanups, fixing things
detected by tools such as go vet, gofmt -s, and golint.
There are no functional changes, this change is purely cosmetic, but will
enable us to run those tools more regularly now that their output is clean.
Add a new module for getting listener sockets via systemd's file descriptor
passing (see sd_listen_fds(3) for more details), and make the main daemon use
it when "systemd" is given an address.
