(PUP-2738) Windows utils appropriate return values

• Puppet::Util::Windows::File.open_symlink was previously returning the integer result of CloseHandle, which was undesireable. Assume the token is always used in a block, so return nil from the method.
• Puppet::Util::Windows::File.readlink was being a bit dangerous, and performing a non-local return within a block. Use a closure instead to capture the return value of resolve_symlink, return outside block.
• Puppet::Util::Windows::Process.open_procss_token had the same issue as open_symlink, and has been changed to return nil.