Services utilising postsrsd need to connect to it via a Unix socket.
While the path to that socket is static as of now, the discoverability
of that path suffers from the lack of module documentation and a
scattered definition of the individual path components over at least
three lines, even when reading the module source.
By exposing the socket path as a readOnly NixOS option, that value shows
up in the options overview and can be re-used in other parts of NixOS
config.
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