The `winpipe` crate which is our current best bet for support on Windows
requires that named pipes are at the magical location `\\.pipe\`.
Provide a default on Windows that matches this, and use our freshly
assigned IANA service name for it.
This is a breaking change, as we remove a `pub const`, it should be
reasonably easy to fix by dependents, and probably it was a mistake to
make the default `pub const` as dependents should call `Home::socket`
anyway.
ce11f03fe6