Remove use of -entry flag for DLLs on Windows

Remove use of -entry flag for DLLs on Windows

Remove the use of the -entry flag to specify a DLL entry point on
Windows, as it is no longer recommended.  The default Windows entry
point will call a DllMain() function if one is provided; use that name
in kfwlogon instead of DllEntryPoint().

[ghudson@mit.edu: edited commit message]