Set old_config_exists to false in Windows if the registry key are not present

[Shivansps]

As i commented in #6392 in Windows during a completely new install on a clean pc FSO will default to legacy config this is because old_config_exists is set to true and never checked, contrary to linux were it checks if there is a config file name or not.

What i found is that if neither the registry keys and cmdline_fso.cfg exists the old_config_time will be 0 as thats is the default "not found" error value for both get_file_modification_time() and os_registry_get_last_modification_time().

But it is not perfect because if you install Freespace 2 from Steam and try to run it, even trought it dosent work, it creates the registry entries, this may be true with the retail cds as well. But it is not the case for the gog version.

But technically the registry entry is there so i dont think i can do anything about it.