make factory function friend and its ctor private

[PeterSommerlad]

Is your feature request related to a problem? Please describe.
public interface of unique_resource should be minimal.

Describe the solution you'd like
the make_unique_resource_checked factory requires a special constructor that should be private and the factory be a friend.

Describe alternatives you've considered
gcc creates an ICE when factory is declared/defined as friend.

Additional context
not sure, how to specify templated factory function as friend correctly within the class template