Various client plugins fixes/improvements

- Datatables now support per value access restrictions. This allows you
to network certain values to players with access to a certain command
only.
- A plugin's Plugin:NetworkUpdate( Key, Old, New ) function will be
called every time a datatable value is changed (if you've made the
function).
- Fixed the sh_loadplugin command not working correctly with shared
plugins.
- Fixed client plugins not being detected after moving the lua files to
lua/shine.