.. and include the core NTLM header in all NTLM-related source files.
Follow up to 6f86022. Since then http_ntlm checks NTLM_NEEDS_NSS_INIT
but did not include vtls.h where it was defined.
Closes https://github.com/curl/curl/pull/1911
Some calls in different modules were setting the data handle to NULL, causing
segmentation faults when using builds that enable character code conversions.
As Windows SSPI authentication calls fail when a particular mechanism
isn't available, introduced these functions for DIGEST, NTLM, Kerberos 5
and Negotiate to allow both HTTP and SASL authentication the opportunity
to query support for a supported mechanism before selecting it.
For now each function returns TRUE to maintain compatability with the
existing code when called.
