Moved the error constant switch to curl_sspi.c and added two new helper
functions to curl_sspi.[ch] which either return the constant or a fully
translated message representing the SSPI security status.
Updated socks_sspi.c and curl_schannel.c to use the new 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.
CURLOPT_SOCKS5_GSSAPI_SERVICE and CURLOPT_SOCKS5_GSSAPI_NEC to allow libcurl
to do GSS-style authentication with SOCKS5 proxies. The curl tool got the
options called --socks5-gssapi-service and --socks5-gssapi-nec to enable
these.