Add support for listening UNIX sockets

[equalsraf]

A new CLI option --socket enables listening on UNIX sockets. This is similar to the --addr option, but takes a path as argument.

If the given path already exists and it is a socket, attempt to remove it before listening.

The port check was refactored to avoid the retrieval of the TCP port on every request.

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Notes:

• The motivation for this was to support running agate over tor, using a unix socket. Feel free to throw away if this does not align with project goals
• Unix sockets are only supported when building in UNIX systems - I think I got the compile time conditionals right - but I did not test in windows.