reverted plugin disabling method back to old behaviour

Ignore-this: b5fadadeee9de8e52c41c056cf62be6d

With this patch plugins are now disabled by placing a disabled file in the
plugin directory again.

Even though renaming plugin directories is the method with the fewest disk
accesses it makes a lot of trouble with code revision control systems and,
more important, Linux package management systems.

Future versions of DokuWiki may use a central config file instead.

This patch also fixes the problem with the plugin manager not checking the
return values of the pugin_(dis|en)able functions correctly.

darcs-hash:20090127182521-7ad00-62018a546d49d57582d93298c8228fd71601a5e8.gz