smtpd.go: expose full TLS connection state in Conn struct, log ServerName

The connection state has enough useful things in it that we should
expose it all rather than doing so piecemeal. This is an API change
since we're removing the TLSCipher field (given that it's just copied
from tls.ConnectionState).

We also log the server name that the remote end sent us on TLS, if
any. Unsurprisingly, it seems that a lot of MTAs don't use SNI and
so this is blank.
sinksmtp: support multiple TLS certificates at once

This adds support for specifying multiple TLS certificates so that
crypto/tls can pick the right one if the client uses SNI. This is
supported on both the command line and in the -conncfg config file.

As a minor side change, sinksmtp also saves the server name that the
client sent us if it did and has been updated for smtpd.go's change in
how TLS connection information is exposed.