Problem: zmsg_popint is not useful (any more)

The usecase I had for this has gone and it's unclear it serves any
purpose. It adds clutter to the API for no benefit.

Solution: remove it.
Problem: still using zlist internally in CZMQ

Solution: leave zlist_t in public API where it's used (e.g. in
zdir and zconfig. Internally, switch all use of zlist to zlistx
except in deprecated classes. Also, switch to zlistx in public
API for zhashx_keys.