FetchContent: Set policies for most commands

Previously, FetchContent was relying on the policies of the
includer being compatible with the module. That made the
module vulnerable to subtle problems where policy settings
might lead to unexpected behavior.

With this change, we now specify the policies for as much of
the module as we can without breaking backward compatibility.
Only a few specific implementation details of the
FetchContent_MakeAvailable() macro have to remain uncontrolled
due to the way those parts can invoke user or project code which
may be relying on the includer's policy settings (intentionally or not).