Problem: new code is duplicated in zsocket and zsock

zsocket is deprecated and there's no reason to add new code to it.
That just creates extra work and maintenance. Those who need this new
functionality can use zsock and the CZMQ v3 API.

Solution: reverted zsocket changes.
Merge pull request #621 from hintjens/master

Problem: new code is duplicated in zsocket and zsock