Both SMTP and LMTP servers may listen either TCP or UNIX domain socket

[ikedas]

• RFC2033 does not state that "LMTP mode cannot be used with a TCP listener": It states that it "MUST NOT be used on the TCP service port 25". In fact, most LMTP implementations support TCP, and the default port number is often 24.
• There is at least one implementation that supports SMTP over UNIX domain socket: OpenSMTPD.
• TLS over UNIX domain socket seems worthwhile. On some systems, since permissions of pathname socket are ignored, they cannot be relied upon (See unix(7)).

That's why this PR makes changes to allow both SMTP and LMTP servers to be used on either TCP or UNIX domain socket with optional TLS.

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Some other changes:

• The server address is treated as UNIX domain socket when the value of Server.Addr contains "/".
• New field Network for the type of connection, defaulting "tcp", is introduced. LMTP field no longer affects the type.
• The default address for ListenAndServeTLS() was changed to ":465" because on several systems ":smtps" cannot be resolved.

[ikedas]

I noticed that a part of the goals of this PR is duplicated in the PR #123 by @patrick246 . However I believe my changes are better.

[emersion]

I am not a fan of making Server.Addr handling more complicated.

[ikedas]

I am not a fan of making Server.Addr handling more complicated.

Is another option like Server.Network preferred? I don't care either way.

[ikedas]

@emersion , I removed auto-detection of network type.
If I should squash the commits, let me know.

[emersion]

Changed the logic a bit, thanks!