NTLM single-sign on adjustments (VIII)

Use preprocessor symbols WINBIND_NTLM_AUTH_ENABLED and WINBIND_NTLM_AUTH_FILE
for Samba's winbind daemon ntlm_auth helper code implementation and filename.

Retain preprocessor symbol USE_NTLM_SSO for NTLM single-sign-on feature
availability implementation independent.

For test harness, prefix NTLM_AUTH environment vars with CURL_

Refactor and rename configure option --with-ntlm-auth to --enable-wb-ntlm-auth[=FILE]