Make config commands understand booleans.

The user frontends for our settings assume everything is a string, so do
the necessary conversions. Note that this means there is no way to set
the string values "true" and "false". For a correct implementation,
users would need to supply types when adding new values.

Change-Id: If3638525eeebe73857834389bae46484eee92cc3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>