Option to use unix sockets instead of TCP

[clarfonthey]

Would be nice to have.

Envisioning this as something like a -u flag where the host is recognised as a directory, and the ports are recognised as files in that directory.

[roehling]

I like this idea, as Postfix added support for socketmap lookups a while ago. It's probably not gonna happen until version 2.0 though, as this is a pretty substantial change, both in the internal logic and the way PostSRSd is integrated with Postfix.

[jeredfloyd]

PR #123 should enable this for the sender rewriting workflow, at least.

[roehling]

https://github.com/roehling/postsrsd/releases/tag/2.0.0a1