Allows display_size to use a fixed unit for easy comparison of
multiple results, and fixed decimal places for the same reason.
Improves behaviour by using consistent decimal places and a
consistent space before the unit (the previous one only has a space
before 'bytes', not before 'KB').
Of existing uses, all the ones that displayed a 'maxbytes' type
configuration setting (which are likely to have an 'exact' size
and would be better shown as 512 KB rather than 512.0 KB) have been
changed to use 0 decimal places, to preserve previous behaviour.
All the uses which were showing an actual file or memory size have
been left as default (1 decimal place).
When defining settings that are used by scheduled tasks,
it is also useful, or even needed, to know the status
of that scheduled task to have the whole big picture of
that part of the system.
Based on the admin_setting_description, this new setting
reports its name, its status, a link to the configuration.
When adding a new setting of this type, the user can add
an extra description field to complete the whole meaning.
sam marshall