Map mod:
Tune cyclic update interval.
Re-add HUD flags update on item 'use'.

Clarify README.txt.

In survival mode, use of the minimap requires the mapping kit item.
Minimap radar mode is always disabled in survival mode.

Minimap and minimap radar mode will be automatically enabled in creative mode
and for any player with the 'creative' privilege.

The 'map.update_hud_flags()' function is global so can be redefined by a mod for
alternative behaviour.