The current code exposes to inconsistent updates, i.e. if handler N succeeds
but handler N+1 fails in setting the property, an error is returned to the
client although parts of the server now behave as if the property change
succeeded.
This patch adds a "checkonly" parameter to the SetProperty handler. The
handlers are then called twice, once with checkonly set to TRUE.
On the checkonly run, handlers _MUST_ return error codes if the property
cannot be applied. Handlers are not permitted to actually apply the changes.
On the second run, handlers are permitted to apply property changes.
Errors codes returned on the second run are ignored.
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