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.
Is this a replication of the logic in `ArgumentHandler`? I.e. the resolution of the log level and debug switch should already have been done by the time this code is run, no?