BDecisionProvider: Make YesNoDecisionNeeded() non-abstract.

- A subset of jobs that require a BContext don't in fact make use of the
decision provider. As such, make the default implementation usable for
those cases so one doesn't need to always create a dummy derived class.

headers/os/package/Context.h

@@ -25,7 +25,7 @@ struct BDecisionProvider {
 	virtual	bool				YesNoDecisionNeeded(const BString& description,
 									const BString& question,
 									const BString& yes, const BString& no,
-									const BString& defaultChoice) = 0;
+									const BString& defaultChoice);
 //	virtual	bool				ActionsAcceptanceDecisionNeeded(
 //									const BString& description,
 //									const BString& question) = 0;