Introduced curl_sspi.c and curl_sspi.h for the implementation of functions

Curl_sspi_global_init() and Curl_sspi_global_cleanup() which previously were
named Curl_ntlm_global_init() and Curl_ntlm_global_cleanup() in http_ntlm.c
Also adjusted socks_sspi.c to remove the link-time dependency on the Windows
SSPI library using it now in the same way as it was done in http_ntlm.c.