Wrap system library on UNIX systems

[tisonkun]

As fast as nix-rust/nix#2537 get released, we can reexport the related defs so that users of fasyslog can directly use them for calling system library to syslog.

This should be guarded behind cfg(unix) but I'd argue that we don't need a new feature flag like native since it's lightweight.

[tisonkun]

I'd consider call a 1.0 release after this gets resolved.

[tisonkun]

Since nix fails to release quickly and anyway a native calls wrapper would be new APIs, I'd try to release 1.0 now.