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