Improve Joomla 1.7 compatibility

• Allow re-installing an extension in a slave site.
  Under joomla 1.7, this functionality may cause a fatal error when trying to re-install an extension from a slave site.
• Also fix the Joomla 1.6 and 1.7 discovery from a slave site.

Bundled with patches definition 1.2.67

• Add several XMap plugins definitions
• Add several sh404SEF plugins definitions