Bring some duplicate code together around the loglevel handling.

Add add_trace_to_loglevel_config and update_loglevel_config.  These two
handle most of the updates to the loglevel config item, including the
update of the overall logging mask.

This makes minimum_loglevel private.

This doesn't change any behavior -- it just a tidy-up.
Add an update to the global loglevel when a new handler is installed.

This fixes a bug where messages were incorrectly discarded if the new
handler is using log levels that are different from those in use by handlers
that were set up at start-of-day.
Merge pull request #114 from ewanmellor/fix-min-level

Add an update to the global loglevel when a new handler is installed