Dmitry Bartsevich discovered some issues in compatibilty of SSPI-enabled

Dmitry Bartsevich discovered some issues in compatibilty of SSPI-enabled
version of libcurl with different Windows versions. Current version of
libcurl imports SSPI functions from secur32.dll. However, under Windows NT
4.0 these functions are located in security.dll, under Windows 9x - in
secur32.dll and Windows 2000 and XP contains both these DLLs (security.dll
just forwards calls to secur32.dll).

Dmitry's patch loads proper library dynamically depending on Windows
version. Function InitSecurityInterface() is used to obtain pointers to all
of SSPI function in one structure.
: ----------------------------------------------------------------------