Implement RFC 4178 (SPNEGO) related extensions

[DirectXMan12]

RFC 4178 contains two methods for use with SPENGO:

• gss_set_neg_mechs
• gss_get_neg_mechs (not implemented in MIT krb5)

These are useful to constrain the mechanisms that SPENGO can use

[optiz0r]

The code in #176 seems to work for me, it would be useful if this could be published as a new release :)

[frozencemetery]

Thanks for the feedback @optiz0r! I'm waiting on Windows support (#177) which should merge soon, and then I'll cut a release.

(Closing ticket because it should have been closed with the merge of #176.)