Problem: not setting a HWM for ZeroMQ/2.x

zsock was not setting a default HWM on older ZeroMQs.

Solution: explicitly set HWM of 1,000 on all new sockets, no matter
the ZeroMQ version. I'll plug this into configurable zsys globals
next.
Problem: cannot configure process global context

Solution: allow configuration of LINGER, IOTHREADS, SNDHWM, RCVHWM,
and IPV6 defaults through the API, using zsys_set_xxx() methods.
Second, allow the underlying defaults to come from the environment
via ZSYS_XXX variables, in same style as ZSYS_INTERFACE.

Provides a consistent configuration interface for zsys.