curl_gssapi: Add macros for common mechs and pass them appropriately

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

That auth mech has never existed neither on MS nor on Unix side.
There is only Negotiate over SPNEGO.
configure/features: Add feature and version info for GSS-API and SPNEGO
curl.h/features: Deprecate GSS-Negotiate 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'
docs: Improve inline GSS-API naming in code documentation