[Patch] add a function to get a SD object from the default security descriptor text value

[Matthieu Patou]

Hello,

Find attach a patch that create a function for the python bindings that 
allow to get a SD from a the textual version of the 
defaultSecurityDescriptor.

This function is needed for the upgradeprovision script so that we can 
work out wether the SD has changed from its default value.

Matthieu.