The signals crate does not build on non-Unix-like platforms, such as
Windows. It uses constants for signals that are not exported from
`libc` on Windows, and transitive dependencies such as `sem_safe` fail
to compile.
Resolve this, by guarding most of the crate, certainly all real
features, by `cfg(unix)`.
To require less uses of the `cfg` macro, move all affected parts into a
new module, and re-export it.
The crate now builds on Windows (although it does not do anything
interesting on that platform) and its Unix interface remains unchanged.
Changes are integrated into `radicle-node`, but only affect Windows.
Lorenz Leutgeb