Add support for modules which can be reloaded but not unloaded.•••This is achieved by replacing the boolean 'norestart' field of the module
header with a tri-state enumeration 'can_unload'. Existing module source using
true/false will still work, since the 0/1 values still have their previous
meanings. However, the module API version has been increased since the size of
the field may have changed.