Syslog calls this level "notice" and I prefer it because we use it
to display significant messages that are not warnings, but they
are not really "standard", as GLib wants them to be. There is nothing
"standard" about log messages in general.
Also, make these notice messages be enabled at debug level 2, together
with warnings. The default log.level is 2 and it is a good idea to show
notices by default too.
Finally, show them in the log with "N" and also change criticals to be
shown with "E", meaning "error"... Then promote G_LOG_LEVEL_ERROR
messages to be shown with "F", meaning "fatal", because in fact these
messages are always fatal and always call abort(). Still, keep the term
"critical" in the functions to make sure that whoever uses them is aware
that this level is only for critical conditions and not suitable to
display any kind of error.
