add support for syslog 6 to fern

[Roguelazer]

See #47

I did this in a way that keeps all syslog-3 and syslog-4 functionality intact.

[BlackDex]

@daboross it would be really great if this could be merged.
This would allow everybody who is using fern together with syslog to get rid of the insecure time crate version!

[BlackDex]

May i suggest that you also update the chrono dependency to omit the oldtime feature?

So instead of:

chrono = { version = "0.4", optional = true }

Use

chrono = { version = "0.4", default-features = false, features = ["clock"], optional = true }

That way the old version of the time crate would be fully gone when using syslog6 🎉

( And after that we only need to hope it gets merged 🤞 )

[daboross]

Thank you for this! This looks great. Sorry to let a good PR sit without a review or merge for so long!

[daboross]

LGTM