curl_gssapi: document/update feature availability

[vszakats]

• update GSS_C_DELEG_POLICY_FLAG comment to include Apple GSS, add
  date, and amend MIT Kerberos version to 1.7+ (was: 1.8+)
  Ref: krb5/krb5@45875a4
  Ref: apple-oss-distributions/Heimdal@1635de3

• document HAVE_GSS_SET_NEG_MECHS/gss_set_neg_mechs().
  Ref: krb5/krb5@079eed2
  It's also committed to Heimdal, but not present in a release
  as of 7.8.0 (current latest).
  Ref: heimdal/heimdal@735039d

Follow-up to a8881e5 #21315 #22410
Follow-up to d169ad6 #22052

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By my estimate Apple forked Heimdal somewhere around 1.4.0 and 1.5.1
heimdal/heimdal@49b471f,
that is around 2011-09, and before 2012-02-03:
apple-oss-distributions/Heimdal@673e1a1

[jay]

I'm kind of surprised gss_set_neg_mechs isn't more places because it's in RFC 4178, 3.2 says ("SHOULD") so I guess it doesn't have to support it?

When a GSS-API credential is acquired for the SPNEGO mechanism, the
implementation SHOULD produce a credential element for the SPNEGO
mechanism that internally contains GSS-API credential elements for
all mechanisms for which the principal has credentials available,
except for any mechanisms that are not to be negotiated, per
implementation-, site-, or application-specific policy.
See Appendix B for interfaces for expressing application policy.

[vszakats]

With GNU GSS, one reason may be that it lacks SPNEGO support.
With Apple GSS, Apple may just have disabled SPNEGO/NTLM in
their fork, that happened around 2011-2012. As for Heimdal, it may
be slow in development and releases.

(According to my guesses looking around the sources.)