Use log levels from rfc5424

[H4ad]

At https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1, page 11.

Suggested by Matteo.

[H4ad]

Blocked by #4

Unblocked, we can just rewrite the new log levels directly.

[H4ad]

@mcollina About this suggestion, these are the log levels:

Numerical       Severity
0               Emergency: system is unusable
1               Alert: action must be taken immediately
2               Critical: critical conditions
3               Error: error conditions
4               Warning: warning conditions
5               Notice: normal but significant condition
6               Informational: informational messages
7               Debug: debug-level messages

Do you think we should keep the log level number as 0-7? Or even allow the users to customize the log level?

I think we maybe will need to be more opinionated here, so probably sticking with 0-7 and also do not allow new log levels.

[H4ad]

About Facility, I think we don't need to introduce that multiplication.

[mcollina]

Keep them fixed for now.

[H4ad]

I updated the current implementation to use these new log levels.

I also added the silent with the value -Infinity.