Added missing ifdef to Curl_http_done if GSS-API or SSPI is not available
Add macros for the most common GSS-API mechs and pass them to
Curl_gss_init_sec_context()

- Macros: KRB5_MECHANISM and SPNEGO_MECHANISM called from
  HTTP, FTP and SOCKS on Unix
Remove checkprefix("GSS-Negotiate")

That auth mech has never existed neither on MS nor on Unix side. There is
only Negotiate with SPNEGO.
Add feature and version info for GSS-API (like with SSPI)
Deprecate GSS-Negotiate related macros due to bad naming

- Replace CURLAUTH_GSSNEGOTIATE with CURLAUTH_NEGOTIATE
- CURL_VERSION_GSSNEGOTIATE is deprecated which
  is served by CURL_VERSION_SSPI, CURL_VERSION_GSSAPI and
  CURUL_VERSION_SPNEGO now.
- Remove display of feature 'GSS-Negotiate'

No macros were removed but deprecated and replaced with new ones.
Make Negotiate (SPNEGO) auth CLI options and help available only if
GSS-API or SSPI is available (USE_HTTP_NEGOTIATE)
Improve inline GSS-API naming in code documentation