Fixed woodhouse example
Problem: breaking change in API

Switching from 'void *' to 'sock_t *' in zpoller, zstr, zmsg, and
zframe broke some upstream applications. (At least czmq++.)

Solution: switch back to using 'void *', and use casts internally
to detect sock_t objects being passed.

At later date we may migrate this using e.g. a macro like CZMQ_SOCK
which the caller can redefine for compatibility.