Move the implementation of _winreg_RestoreKey to reg_api.c

Move the implementation of _winreg_RestoreKey to reg_api.c

This removes the internals of reg_api from srv_winreg_nt.c entirely,
only reg_api is used there, now. This enlarges the dependencies of
reg_api somewhat now since it adds regfio. But this can be separated
out later. The current goal is to achieve a complete use of reg_api.

Michael