Allow logging to specify an explicit log level instead of a macro

For log entries which may have a variable level, we can't call an
arbitrary macro and need to be able to pass an explicit level. This
does so without breaking the compile-time disabling of certain log
levels.

Further, we "fix" the comparison order of log levels to make more
significant levels sort "higher", which implicitly makes more sense
than sorting "lower".

Finally, we remove the "Off" log level as no log entry should ever
be logged at the "Off" level - that would be nonsensical.