Problem: ZMQ_MSG_T_SIZE draft but not in DRAFT section

Solution: move it there, add it to src/zmq_draft.h and clarify in the
documentation and in the NEWS file that this option is not yet stable

NEWS

 0MQ version 4.2.1 stable, released on 201x/xx/xx
 =============================================
 
-* New Context options:
+* New DRAFT (see NEWS for 4.2.0) Context options:
   - ZMQ_MSG_T_SIZE
 
 

doc/zmq_ctx_get.txt

@@ -62,6 +62,7 @@ ZMQ_MSG_T_SIZE: Get the zmq_msg_t size at runtime
 The 'ZMQ_MSG_T_SIZE' argument returns the size of the zmq_msg_t structure at
 runtime, as defined in the include/zmq.h public header.
 This is useful for example for FFI bindings that can't simply do a sizeof().
+NOTE: in DRAFT state, not yet available in stable releases.
 
 
 RETURN VALUE

include/zmq.h

@@ -213,9 +213,6 @@ ZMQ_EXPORT void zmq_version (int *major, int *minor, int *patch);
 #define ZMQ_THREAD_PRIORITY 3
 #define ZMQ_THREAD_SCHED_POLICY 4
 #define ZMQ_MAX_MSGSZ 5
-//  All options after this is for version 4.3 and still *draft*
-//  Subject to arbitrary change without notice
-#define ZMQ_MSG_T_SIZE 6
 
 /*  Default for new contexts                                                  */
 #define ZMQ_IO_THREADS_DFLT  1
@@ -371,8 +368,6 @@ ZMQ_EXPORT const char *zmq_msg_gets (zmq_msg_t *msg, const char *property);
 #define ZMQ_VMCI_BUFFER_MAX_SIZE 87
 #define ZMQ_VMCI_CONNECT_TIMEOUT 88
 #define ZMQ_USE_FD 89
-//  All options after this is for version 4.3 and still *draft*
-//  Subject to arbitrary change without notice
 
 /*  Message options                                                           */
 #define ZMQ_MORE 1
@@ -566,6 +561,9 @@ ZMQ_EXPORT void zmq_threadclose (void* thread);
 #define ZMQ_SCATTER 17
 #define ZMQ_DGRAM 18
 
+/*  DRAFT Context options                                                           */
+#define ZMQ_MSG_T_SIZE 6
+
 /*  DRAFT Socket methods.                                                     */
 ZMQ_EXPORT int zmq_join (void *s, const char *group);
 ZMQ_EXPORT int zmq_leave (void *s, const char *group);

src/zmq_draft.h

@@ -46,6 +46,9 @@
 #define ZMQ_SCATTER 17
 #define ZMQ_DGRAM 18
 
+/*  DRAFT Context options                                                           */
+#define ZMQ_MSG_T_SIZE 6
+
 /*  DRAFT Socket methods.                                                     */
 int zmq_join (void *s, const char *group);
 int zmq_leave (void *s, const char *group);

tests/test_ctx_options.cpp

@@ -48,7 +48,9 @@ int main (void)
 #endif
     assert (zmq_ctx_get (ctx, ZMQ_IO_THREADS) == ZMQ_IO_THREADS_DFLT);
     assert (zmq_ctx_get (ctx, ZMQ_IPV6) == 0);
+#if defined (ZMQ_BUILD_DRAFT_AP)
     assert (zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) == sizeof (zmq_msg_t));
+#endif
     
     rc = zmq_ctx_set (ctx, ZMQ_IPV6, true);
     assert (zmq_ctx_get (ctx, ZMQ_IPV6) == 1);