833 Allow log level to be set in debug mode.

Fixes #833 by allowing any log level to be set in debug mode. If this is accepted then it makes #871 obsolete.

@Romoku, do you want to see if this does what you hope it will? You should run with `-D -loglevel info` or `error` if you do not want to see all of the `debug` output.
OK, fair points. How about a compromise: this line simply defaults to info level when null, but the resolution of log level and debug switch is left in `ArgumentHandler` where it belongs? I just want to avoid the duplication.