gssapi: move new options to DRAFT section

Problem: The new GSSAPI NAMESPACE options should have been
added to the DRAFT section of the API so they can be changed
until stabilized.

Solution:
- Move defines to the DRAFT section of zmq.h
- Duplicate them in zmq_draft.h, as is the local custom
- Compile only if defined (ZMQ_BUILD_DRAFT_API)
- Refactor internals slightly to avoid #ifdef hell

src/options.cpp

@@ -69,8 +69,10 @@ zmq::options_t::options_t () :
     tcp_keepalive_intvl (-1),
     mechanism (ZMQ_NULL),
     as_server (0),
+#ifdef ZMQ_BUILD_DRAFT_API
     gss_principal_nt (ZMQ_GSSAPI_NT_HOSTBASED),
     gss_service_principal_nt (ZMQ_GSSAPI_NT_HOSTBASED),
+#endif
     gss_plaintext (false),
     socket_id (0),
     conflate (false),
@@ -511,6 +513,7 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
                 return 0;
             }
             break;
+#ifdef ZMQ_BUILD_DRAFT_API
         case ZMQ_GSSAPI_PRINCIPAL_NAMETYPE:
             if (is_int && (value == ZMQ_GSSAPI_NT_HOSTBASED
                         || value == ZMQ_GSSAPI_NT_USER_NAME
@@ -527,6 +530,7 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
                 return 0;
             }
             break;
+#endif
 #endif
 
         case ZMQ_HANDSHAKE_IVL: