gss: port NegoEx implementation from MIT

An implementation of draft-zhu-negoex-04 for MIT Kerberos was developed in
2011. This has been recently integrated, with many fixes from Greg Hudson. This
commit ports it to Heimdal. The implementation has been interoperability tested
with MIT Kerberos and Windows, using the GSS EAP mechanism developed as part of
the Moonshot project.

The SPNEGO code was also updated to import the state machine from Apple which
improves mechListMIC processing and avoids discarding initial context tokens
generated during mechanism probing, that can be used for optimistic tokens.

Finally, to aid in testing, the GSS-API mechanism glue configuration file can
be changed using the environment variable GSS_MECH_CONFIG. This environment
variable name, along with the format of the configuration file, is compatible
with MIT (although it would be difficult for a single mechanism binary to
support both implementations).

doc/setup.texi

@@ -121,6 +121,12 @@ can point a file with the environment variable @samp{KRB5_CONFIG}.
 env KRB5_CONFIG=$HOME/etc/krb5.conf kinit user@@REALM
 @end example
 
+@cindex GSS_MECH_CONFIG
+The GSS-API mechanism configuration file can also be changed from the
+default with the enviornment variable @samp{GSS_MECH_CONFIG}. Note that
+this file only configures additional plugin mechanisms: Kerberos, NTLM
+and SPNEGO are built in to the Heimdal GSS-API library.
+
 @node Creating the database, Modifying the database, Configuration file, Setting up a realm
 @section Creating the database