journald: Map "info" level to journald "info" priority

[hongquan]

Motivation

In the current implementation, "info" level is mapped to Journald "notice" priority. This mapping is not reasonable, because, according to https://en.wikipedia.org/wiki/Syslog#Severity_level:

notice: Conditions that are not error conditions, but that may require special handling

and in reality, journactl displays "notice" message as bold text, in constrast to normal style of "info".
So this PR is to remap "info" tracing level to Journald "info" priority.

Solution

Remap:

info => info
debug | tracing => debug.

[hongquan]

@hawkw Could you take a look?

[glts]

Noting in passing that there is also pull request #2347, which would make the mapping freely configurable.

[hawkw]

I would prefer to merge #2347, which (as @glts pointed out) is a more flexible solution. If the original contributor of that change doesn't have the opportunity to address the last feedback I left, I'm willing to go ahead and merge that PR as-is.

[Ralith]

Closing as we'd rather make this configurable. I do think our current behavior is the correct default: it retains distinctions between all tracing levels, and it's reasonable for debug! messages to be unremarkable whereas info! might have special significance.