Lock down the search path for Windows DLL loading.

Lock down the search path for Windows DLL loading.

At least on systems providing SetDefaultDllDirectories, this should
stop PuTTY from being willing to load DLLs from its containing
directory - which makes no difference when it's been properly
installed (in which case the application dir contains no DLLs anyway),
but does if it's being run from somewhere uncontrolled like a browser
downloads directory.

Preliminary testing suggests that this shouldn't break any existing
deliberate use of DLLs, including GSSAPI providers.