kdc: support for GSS-API pre-authentication

Add support for GSS-API pre-authentication to the KDC, using a simplified
variation of draft-perez-krb-wg-gss-preauth-02 that encodes GSS-API context
tokens directly in PADATA, and uses FX-COOKIE for state management.

More information on the protocol and implementation may be found in
lib/gssapi/preauth/README.md.

lib/gssapi/mech/gss_mech_switch.c

@@ -563,3 +563,16 @@ gss_oid_to_name(gss_const_OID oid)
 
 	return NULL;
 }
+
+GSSAPI_LIB_FUNCTION uintptr_t GSSAPI_CALLCONV
+gss_get_instance(const char *libname)
+{
+    static const char *instance = "libgssapi";
+
+    if (strcmp(libname, "gssapi") == 0)
+	return (uintptr_t)instance;
+    else if (strcmp(libname, "krb5") == 0)
+	return krb5_get_instance(libname);
+
+    return 0;
+}